MarkWest Energy Finance 424B5 Prospectus supplement for primary offering

Use these links to rapidly review the document
TABLE OF CONTENTS
Table of Contents

CALCULATION OF REGISTRATION FEE

Class of securities registered	Maximum Aggregate Offering Price	Amount of Registration Fee




4.875% Senior Notes due 2025	$1,200,000,000	$139,440

(1): The filing fee, calculated in accordance with Rule 457(r), has been transmitted to the SEC in connection with the securities offered from Registration Statement File No. 333-184605 by means of this prospectus supplement.

Table of Contents

Filed Pursuant to rule 424(b)(5)
Registration File No. 333-184605

PROSPECTUS SUPPLEMENT
(To Prospectus dated October 26, 2012)

$1,200,000,000

MarkWest Energy Partners, L.P.
MarkWest Energy Finance Corporation
4.875% Senior Notes due 2025

MarkWest Energy Partners, L.P. and MarkWest Energy Finance Corporation are offering $1,200,000,000 aggregate principal amount of 4.875% Senior Notes due 2025 (the "notes").

The notes will mature on June 1, 2025. Interest on the notes will accrue from June 2, 2015, and we will pay interest on the notes twice a year, beginning on December 1, 2015.

We may redeem, at our option, some or all of the notes at any time at the redemption prices described in this prospectus supplement under "Description of Notes — Optional Redemption." If we sell certain of our assets or experience specific kinds of changes of control, we must offer to repurchase the notes.

The notes will be the senior unsecured obligations of MarkWest Energy Partners, L.P. and MarkWest Energy Finance Corporation, a wholly-owned subsidiary of ours that has no material assets and was formed for the sole purpose of being a co-issuer of some of our indebtedness, including the notes. The notes will rank equally with all of our existing and future senior debt, senior to all of our future subordinated debt and effectively junior in right of payment to all of our secured debt to the extent of the value of the collateral securing such indebtedness. Further, the notes will be guaranteed on a senior unsecured basis by each of our existing subsidiaries that guarantees both our revolving credit facility and our outstanding senior notes and by certain of our future subsidiaries. The notes will be structurally subordinated to the liabilities of any of our subsidiaries that do not guarantee the notes.

Investing in the notes involves risks. Please read "Risk Factors" beginning on page S-8.

			Total

Initial price to public(1)	99.02600	%	$	1,188,312,000
Underwriting discounts and commissions	0.95833	%	$	11,500,000
Proceeds, before expenses, to us(1)	98.06767	%	$	1,176,812,000

(1): Plus accrued interest, if any, from June 2, 2015.

None of the Securities and Exchange Commission, any state securities commission or any other regulatory body has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying base prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The underwriters expect that the delivery of the notes will be made to investors in book-entry form through the facilities of The Depository Trust Company on or about June 2, 2015.


Joint Book-Running Managers
Wells Fargo Securities	Barclays	BofA Merrill Lynch
Citigroup	Goldman, Sachs & Co.	J.P. Morgan
Morgan Stanley	RBC Capital Markets	SunTrust Robinson Humphrey
UBS Investment Bank		US Bancorp

Senior Co-Managers
BBVA	BNP PARIBAS	Capital One Securities
Comerica Securities	Natixis	PNC Capital Markets LLC
	SMBC Nikko

Prospectus Supplement dated May 28, 2015.

Table of Contents

ABOUT THIS PROSPECTUS SUPPLEMENT

This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of the notes. The second part is the accompanying base prospectus, which gives more general information, some of which may not apply to this offering. Generally, when we refer only to the "prospectus," we are referring to both parts combined. If the information about this offering varies between this prospectus supplement and the accompanying base prospectus, you should rely on the information in this prospectus supplement.

Any statement made in this prospectus or in a document incorporated or deemed to be incorporated by reference into this prospectus will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained in this prospectus or in any other subsequently filed document that is also incorporated by reference into this prospectus modifies or supersedes that statement. Any statement so modified or superseded will not be deemed, except as so modified or superseded, to constitute a part of this prospectus. Please read "Where You Can Find More Information" on page S-67 of this prospectus supplement.

You should rely only on the information contained in or incorporated by reference into this prospectus supplement, the accompanying base prospectus and any free writing prospectus relating to this offering of the notes. Neither we nor the underwriters have authorized anyone to provide you with additional or different information. If anyone provides you with additional, different or inconsistent information, you should not rely on it. We are offering to sell the notes, and seeking offers to buy the notes, only in jurisdictions where such offers and sales are permitted. You should not assume that the information contained in this prospectus supplement, the accompanying base prospectus or any free writing prospectus is accurate as of any date other than the dates shown in these documents or that any information we have incorporated by reference herein is accurate as of any date other than the date of any document incorporated by reference. Our business, financial condition, results of operations and prospects may have changed since such dates.

FORWARD-LOOKING STATEMENTS

Statements included or incorporated by reference in this prospectus supplement that are not historical facts are forward-looking statements. We use words such as "could," "may," "predict," "should," "expect," "hope," "continue," "potential," "plan," "intend," "anticipate," "project," "believe," "estimate," and similar expressions to identify forward-looking statements.

These statements are based on current expectations, estimates, assumptions and beliefs concerning future events impacting us and therefore involve a number of risks and uncertainties. Forward-looking statements are not guarantees and actual results could differ materially from those expressed or implied in the forward-looking statements as a result of a number of factors. We do not update publicly any forward-looking statement with new information or future events. Undue reliance should not be placed on forward-looking statements as many of these factors are beyond our ability to control or predict. Please read "Risk Factors."

S-i

Table of Contents

TABLE OF CONTENTS

Prospectus Supplement


ABOUT THIS PROSPECTUS SUPPLEMENT		S-i
FORWARD-LOOKING STATEMENTS		S-i
SUMMARY		S-1
OUR STRUCTURE AND MANAGEMENT		S-3
RISK FACTORS		S-8
USE OF PROCEEDS		S-11
CAPITALIZATION		S-12
RATIO OF EARNINGS TO FIXED CHARGES		S-14
DESCRIPTION OF NOTES		S-15
BOOK-ENTRY, DELIVERY AND FORM		S-57
DESCRIPTION OF OTHER INDEBTEDNESS		S-60
CERTAIN U.S. FEDERAL INCOME TAX CONSEQUENCES		S-61
CERTAIN ERISA CONSIDERATIONS		S-66
UNDERWRITING		S-68
VALIDITY OF THE NOTES		S-72
EXPERTS		S-72
WHERE YOU CAN FIND MORE INFORMATION		S-72

Prospectus


ABOUT THIS PROSPECTUS		1
WHERE YOU CAN FIND MORE INFORMATION		2
FORWARD-LOOKING STATEMENTS		4
ABOUT MARKWEST ENERGY PARTNERS		5
RISK FACTORS		6
USE OF PROCEEDS		7
RATIOS OF EARNINGS TO FIXED CHARGES		8
DESCRIPTION OF COMMON UNITS		9
DESCRIPTION OF OUR DEBT SECURITIES		11
CASH DISTRIBUTION POLICY		20
PARTNERSHIP AGREEMENT		21
MATERIAL TAX CONSEQUENCES		31
INVESTMENT IN MARKWEST ENERGY PARTNERS BY EMPLOYEE BENEFIT PLANS		45
PLAN OF DISTRIBUTION		48
VALIDITY OF THE SECURITIES		50
EXPERTS		50

S-ii

Table of Contents

SUMMARY

This summary highlights information contained elsewhere in this prospectus supplement and the accompanying base prospectus. It does not contain all of the information that you should consider before making an investment decision. For a more complete understanding of this offering, you should read this entire prospectus supplement, the accompanying base prospectus and the documents incorporated herein and therein by reference, including our historical financial statements and the notes thereto, which are incorporated herein by reference to our annual report on Form 10-K for the year ended December 31, 2014 and any subsequent reports on Form 10-Q. Please read "Where You Can Find More Information" on page S-67 of this prospectus supplement. Please read "Risk Factors" beginning on page S-8 of this prospectus supplement and Part I — Item 1A in our annual report on Form 10-K for the year ended December 31, 2014 and any subsequent quarterly reports on Form 10-Q which are incorporated herein by reference for information regarding risks you should consider before buying the notes in this offering.

For purposes of this prospectus supplement and the accompanying base prospectus, unless the context clearly indicates otherwise, "we," "us," "our" and the "Partnership" refer to MarkWest Energy Partners, L.P. and, when applicable, its consolidated subsidiaries; and "MarkWest Hydrocarbon" refers to MarkWest Hydrocarbon, Inc. and its direct and indirect consolidated subsidiaries.

MarkWest Energy Partners, L.P.

We are a publicly traded master limited partnership that owns and operates midstream services related businesses. We have a leading presence in many natural gas resource plays including the Marcellus Shale, Utica Shale, Huron/Berea Shale, Haynesville Shale, Woodford Shale and Granite Wash formation where we provide midstream services to our producer customers.

Our integrated midstream energy asset network links producers of natural gas, natural gas liquids ("NGLs") and crude oil from some of the largest supply basins in the United States to domestic and international markets. Our midstream energy operations include: natural gas gathering, processing and transportation; NGL gathering, transportation, fractionation, storage, and marketing; and crude oil gathering and transportation. Our assets include approximately 5,800 MMcf/d of natural gas processing capacity, 379,000 Bbl/d of NGL fractionation capacity and over 4,000 miles of pipelines.

We conduct our operations in the following operating segments: Marcellus, Utica, Northeast and Southwest. Further description of these segments is included in the periodic reports we file with the Securities and Exchange Commission ("SEC"), which are incorporated herein by reference. Our primary business strategy is to provide best-in-class midstream services by developing and operating high-quality, strategically-located assets in liquids-rich resource plays in the United States.

Execution of our business strategy has allowed us to grow substantially since our inception. As a result, we are now a leading provider of gathering, processing and fractionation services in the United States.

Ongoing Acquisition Activities

Consistent with our business strategy, we are continuously engaged in discussions with potential sellers regarding the possible purchase of assets and operations that are strategic and complementary to our existing operations. In addition, we have in the past evaluated and pursued, and intend in the future to evaluate and pursue, other energy-related assets that have characteristics and opportunities similar to our existing business lines and enable us to leverage our asset base, knowledge base and skill sets. Such acquisition efforts may involve participation by us in processes that have been made public and involve a number of potential buyers, commonly referred to as "auction" processes, as well as situations in which we believe we are the only party or one of a limited number of potential buyers in negotiations with the potential seller. These acquisition efforts often involve assets which, if acquired, could have a material effect on our financial condition and results of operations.

S-1

Table of Contents

We typically announce a transaction after we have executed a definitive acquisition agreement. However, in certain cases in order to protect our business interests or for other reasons, we may defer public announcement of an acquisition until closing or a later date. Past experience has demonstrated that discussions and negotiations regarding a potential acquisition can advance or terminate in a short period of time. Moreover, the closing of any transaction for which we have entered into a definitive acquisition agreement will be subject to customary and other closing conditions, which may not ultimately be satisfied or waived. Accordingly, we can give no assurance that our current or future acquisition efforts will be successful. Although we expect any acquisitions we make to be accretive in the long term, we can provide no assurance that our expectations will ultimately be realized.

For the three months ended March 31, 2015, our share of total capital spent to fund identified organic growth, expansion projects, acquisitions, and maintenance expenditures on our businesses was approximately $468.9 million.

Tender Offers

On May 28, 2015, we commenced tender offers (the "Tender Offers") to purchase for cash, subject to certain conditions, any and all of the $500 million outstanding principal amount of our 6.75% senior notes due 2020 (the "2020 Notes"), $325 million outstanding principal amount of our 6.5% senior notes due 2021 (the "2021 Notes") and $455 million outstanding principal amount of our 6.25% senior notes due 2022 (the "2022 Notes").

Pursuant to the Tender Offers, we are offering to purchase any and all of the 2020 Notes, 2021 Notes and 2022 Notes tendered prior to the expiration of the Tender Offers for consideration of $1,058.14 per $1,000 principal amount of the 2020 Notes, $1,072.38 per $1,000 principal amount of the 2021 Notes and $1,111.63 per $1,000 principal amount of the 2022 Notes. Each Tender Offer is scheduled to expire at 5:00 p.m., New York City time, on June 3, 2015 and is subject to the satisfaction or waiver of certain conditions, including our receiving net proceeds from this offering in an amount sufficient to fund all of our obligations under the Tender Offers. Provided that the conditions to the Tender Offers have been satisfied or waived, we will pay for the 2020 Notes, 2021 Notes and 2022 Notes accepted for purchase in the Tender Offers, together with any accrued and unpaid interest thereon, promptly after the expiration of the Tender Offers, which is expected to be June 4, 2015 (or June 8, 2015 in the case of notes tendered pursuant to a Notice of Guaranteed Delivery).

This offering is not conditioned upon our completion of any of the Tender Offers. If any condition of any of the Tender Offers is not satisfied or waived, we will not be obligated to accept for purchase, or to pay for, any of the notes subject to such Tender Offer and may delay acceptance for payment of any tendered notes, in each case subject to applicable laws. We may also terminate, extend or amend the terms of any of the Tender Offers and may postpone the acceptance for purchase of, and payment for, the notes tendered. This prospectus supplement is not an offer to purchase the 2020 Notes, 2021 Notes or 2022 Notes. The Tender Offers are made only by and pursuant to the terms of an Offer to Purchase and the related Letter of Transmittal and Notice of Guaranteed Delivery, each dated May 28, 2015, as the same may be amended or supplemented.

If fully subscribed by June 4, 2015, we expect that the Tender Offers will result in an after-tax loss from repurchase of the 2020 Notes, 2021 Notes and 2022 Notes of approximately $111.3 million, of which, approximately $103.5 million relates to the payment of tender premiums and third party expenses (but excluding accrued and unpaid interest), which would be funded with a portion of the net proceeds from this offering as described in "Use of Proceeds."

There is no assurance that the Tender Offers will be subscribed for in any amount. Depending upon the results of a Tender Offer, we may or may not elect to redeem any of the notes that were subject to that Tender Offer but remain outstanding afterwards; and, if we do elect to redeem any such notes, we may redeem them promptly after consummating the Tender Offer or at one or more later times, including on or after the first call date for the applicable series of notes. If we elect to redeem any notes of one series in such circumstances, we may or may not elect to redeem any notes of the other series.

S-2

Table of Contents

Our Structure and Management

The diagram below depicts our organization and ownership structure as of May 26, 2015, without giving effect to our Class A units issued in connection with our acquisition of MarkWest Hydrocarbon, Inc. in 2008. All of our Class B Units are owned by an affiliate of The Energy & Minerals Group ("EMG") and are included in the public ownership percentage in the diagram below. The Class B Units were issued in December 2011 in connection with our acquisition of EMG's interest in MarkWest Liberty Midstream & Resources, L.L.C. ("Liberty").

MarkWest Energy Finance Corporation, the co-issuer of the notes, is a direct wholly owned subsidiary of MarkWest Energy Partners and carries on no independent business other than acting as a co-issuer of the notes and our other senior notes.

For more details about our ownership structure, please read "Organizational Structure" included in Item 1 in our annual report on Form 10-K for the year ended December 31, 2014, which is incorporated herein by reference.

Our Management

Our general partner, which we indirectly own and control, has sole responsibility for conducting our business and for managing our operations. Our common unitholders have the right to elect the members of the board of directors of our general partner at our annual meeting of unitholders.

Principal Executive Offices and Internet Address

Our principal executive offices are located at 1515 Arapahoe Street, Tower 1, Suite 1600, Denver, Colorado 80202. We maintain a website at http://www.markwest.com. The information on our website is not part of this prospectus supplement, and you should rely only on information contained in or

S-3

Table of Contents

incorporated by reference herein and any free writing prospectus filed in connection with this offering when making an investment decision.

Additional Information

For additional information about us, including our partnership structure and management, please refer to the documents set forth under "Where You Can Find More Information," including our annual report on Form 10-K for the fiscal year ended December 31, 2014 and any subsequent quarterly reports on Form 10-Q which are incorporated by reference herein.

S-4

Table of Contents

THE OFFERING


The issuers		MarkWest Energy Partners, L.P. and MarkWest Energy Finance Corporation.
Securities offered		$1,200,000,000 aggregate principal amount of 4.875% Senior Notes due 2025 (the "notes").
Maturity date		June 1, 2025.
Interest payment dates		June 1 and December 1 of each year, commencing December 1, 2015. Interest on the notes will accrue from June 2, 2015.
Optional redemption		We may redeem, at our option, all or part of the notes at any time, prior to March 1, 2025 (three months prior to their maturity date) at the applicable redemption prices described under "Description of Notes — Optional Redemption" plus accrued interest to the date of redemption. We may also redeem, at our option, all or part of the notes at any time on or after March 1, 2025 (three months prior to their maturity date), at a price of 100% of the principal amount thereof plus accrued interest to the date of redemption.
Mandatory offer to repurchase		If we sell certain assets and do not reinvest the proceeds or repay senior indebtedness, or if we experience specific kinds of change of control events, we must offer to repurchase the notes at the prices, in the amounts and subject to the conditions described in the section "Description of Notes — Repurchase at the Option of Holders."
Subsidiary guarantees		Each of our existing subsidiaries that guarantees our debt under both our revolving credit facility (the "Credit Facility") and our outstanding series of senior notes will guarantee the notes initially. If we cannot make payments on the notes when they are due, the guarantor subsidiaries, if any, must make them instead. Certain of our existing subsidiaries are not guarantors, and not all of our future subsidiaries will have to become guarantors. Please read "Description of Notes — The Guarantees."
Ranking		The notes will be:
		• our senior unsecured obligations;
		• equal in right of payment with all of our existing and future senior debt;
		• senior in right of payment to all of our future subordinated debt;
		• effectively junior in right of payment to our secured debt to the extent of the value of the assets securing the debt, including our obligations in respect of the Credit Facility; and
		• structurally subordinated to all liabilities of our subsidiaries that do not guarantee the notes.

S-5

Table of Contents


		Upon the closing of this offering and the application of the net proceeds therefrom in the manner described under "Use of Proceeds," we anticipate that we and the subsidiary guarantors will have approximately $4.2 billion of consolidated indebtedness (including the notes offered hereby) ranking equally in right of payment with the notes and the guarantees. As of May 26, 2015, we and the subsidiary guarantors had approximately $120.2 million of secured indebtedness outstanding (excluding obligations under letters of credit and hedges outstanding under the Credit Facility).
Covenants		We will issue the notes under an indenture with Wells Fargo Bank, National Association, as trustee. The indenture, among other things, will limit our ability and the ability of our restricted subsidiaries to:
		• borrow money;
		• pay distributions or dividends on equity or purchase, redeem or otherwise acquire equity;
		• make investments;
		• use assets as collateral in other transactions;
		• sell certain assets or merge with or into other companies; and
		• engage in transactions with affiliates.
		For more details, please read "Description of Notes — Covenants."
Covenant termination		If at any time the notes are rated investment grade by either Standard & Poor's Ratings Services ("Standard & Poor's") or Moody's Investors Service, Inc. ("Moody's") and no default has occurred and is continuing under the indenture, certain of the foregoing covenants will terminate and will no longer apply to us or our subsidiaries. Please read "Description of Notes — Covenants — Covenant Termination."
Listing for trading		We do not intend to list the notes for trading on any securities exchange. We can provide no assurance as to the liquidity of, or development of any trading market for, the notes.
Use of proceeds		We intend to use the net proceeds from this offering to fund the payments we will make upon the settlement of our pending Tender Offers for any and all of our outstanding 2020 Notes, 2021 Notes and 2022 Notes. Please read "Summary — Tender Offers." Any amounts required to make the payments necessary at settlement of the Tender Offers that are not funded by the net proceeds from this offering will be funded partly by cash on hand, with the remainder to be funded through borrowings under the Credit Facility. Please read "Use of Proceeds."

S-6

Table of Contents


		Certain of the underwriters or their affiliates may be holders of notes subject to the Tender Offers and may therefore also receive a portion of the proceeds from this offering in respect of such notes. Please read "Underwriting."
Risk factors		Investing in the notes involves risks. Please read "Risk Factors" beginning on page S-8 of this prospectus supplement and in the documents incorporated by reference, as well as the other cautionary statements throughout this prospectus, for a discussion of factors you should carefully consider before deciding to invest in the notes.

S-7

Table of Contents

RISK FACTORS

An investment in the notes is subject to a number of risks. You should carefully consider the risk factors included below and those in Item 1A. "Risk Factors" in our annual report on Form 10-K for the year ended December 31, 2014 and any subsequent quarterly reports on Form 10-Q which are incorporated herein by reference, together with all of the other information included or incorporated by reference in this prospectus supplement. You should consider the following risk factors in evaluating this investment. If any of these risks materialize into actual events, our business, financial condition, results of operations or cash flow could be adversely affected.

Risks Related to the Notes

We have a holding company structure in which our subsidiaries conduct our operations and own our operating assets.

We are a holding company, and our subsidiaries conduct all of our operations and own all of our operating assets. We have no significant assets other than the partnership interests, stock and the other equity interests in our subsidiaries. As a result, our ability to make required payments on the notes depends on the performance of our subsidiaries and their ability to distribute funds to us. The ability of our subsidiaries to make distributions to us may be restricted by, among other things, the Credit Facility and applicable state business organization laws and other laws and regulations. If we are unable to obtain the funds necessary to pay the principal amount at maturity of the notes, or to repurchase the notes upon the occurrence of a change of control, we may be required to adopt one or more alternatives, such as a refinancing of the notes. We cannot assure you that we would be able to refinance the notes or obtain the funds to pay principal or interest on the notes.

Payment of principal and interest on the notes will be effectively subordinated to our senior secured debt to the extent of the value of the assets securing that debt and structurally subordinated to the liabilities of any of our subsidiaries that do not guarantee the notes.

The notes will be effectively subordinated to claims of our secured creditors, and the subsidiary guarantees will be effectively subordinated to the claims of our secured creditors as well as the secured creditors of the subsidiary guarantors. As of May 26, 2015, we and the subsidiary guarantors had approximately $120.2 million of secured indebtedness outstanding (excluding obligations under letters of credit and hedges outstanding under the Credit Facility). Holders of our secured obligations, including obligations under the Credit Facility, will have claims that are prior to claims of the holders of the notes with respect to the assets securing those obligations. In the event of a liquidation, dissolution, reorganization, bankruptcy or any similar proceeding, our assets and those of our subsidiaries will be available to pay obligations on the notes and the guarantees only after holders of our senior secured debt have been paid the value of the assets securing such debt. Although almost all of our wholly owned subsidiaries will guarantee the notes, in the future, under certain circumstances, these guarantees are subject to release, and we have various partially owned subsidiaries that will not guarantee the notes. Consequently, the notes will be structurally subordinated to the claims of all creditors, including trade creditors and tort claimants, of our subsidiaries that are not guarantors. In the event of the liquidation, dissolution, reorganization, bankruptcy or similar proceeding of the business of a subsidiary that is not a guarantor, creditors of that subsidiary would generally have the right to be paid in full before any distribution is made to us or the holders of the notes. Accordingly, there may not be sufficient funds remaining to pay amounts due on all or any of the notes.

We require a significant amount of cash to service our indebtedness. Our ability to generate cash depends on many factors beyond our control.

Our ability to make payments on and to refinance our indebtedness, including the notes, and to fund planned capital expenditures depends on our ability to generate cash in the future. This, to a

S-8

Table of Contents

certain extent, is subject to general economic, financial, competitive, legislative, regulatory and other factors that are beyond our control.

We cannot assure you that we will generate sufficient cash flow from operations or that future borrowings will be available to us under the Credit Facility or otherwise in an amount sufficient to enable us to pay our indebtedness, including the notes, or to fund our other liquidity needs. We may need to refinance all or a portion of our indebtedness, including the notes, on or before maturity. We cannot assure you that we will be able to refinance any of our indebtedness, including indebtedness outstanding under the Credit Facility and the notes, on commercially reasonable terms or at all.

We distribute all of our available cash to our unitholders and we are not required to accumulate cash for the purpose of meeting our future obligations to our noteholders, which may limit the cash available to service the notes.

Subject to the limitations on restricted payments contained in the indenture governing the notes, the credit agreement governing the Credit Facility and other indebtedness, we distribute all of our "available cash" each quarter to our limited partners and our general partner. "Available cash" is defined in our partnership agreement, and it generally means, for each fiscal quarter:

•

all cash and cash equivalents on hand at the end of the quarter (excluding cash at MarkWest Hydrocarbon);

•

less the amount of cash that our general partner determines in its reasonable discretion is necessary or appropriate to:

•: provide for the proper conduct of our business;
•: comply with applicable law, any of our debt instruments, or other agreements; or
•: provide funds for distributions to unitholders for any one or more of the next four quarters;

•

plus all cash and cash equivalents on hand on the date of determination of available cash for the quarter resulting from working capital borrowings made after the end of the quarter. Working capital borrowings are generally borrowings that are made under the Credit Facility and in all cases are used solely for working capital purposes or to pay distributions to partners.

As a result, we do not expect to accumulate significant amounts of cash. Depending on the timing and amount of our cash distributions, these distributions could significantly reduce the cash available to us in subsequent periods to make payments on the notes.

We may not be able to fund a change of control offer.

Upon the occurrence of certain change of control events, we will be required, subject to certain conditions, to offer to purchase all outstanding notes at a price equal to 101% of the principal amount thereof, plus accrued and unpaid interest thereon to the date of purchase. The holders of our other outstanding series of senior notes have similar put rights. If a change of control event were to occur today, we would not have sufficient funds available to purchase all of the outstanding notes were they to be tendered in response to an offer made as a result of a change of control event. We cannot assure you that we will have sufficient funds available or that we will be permitted by our other debt instruments to fulfill these obligations upon a change of control event in the future. Furthermore, certain change of control events would constitute an event of default under the Credit Facility. Please read "Description of Notes — Repurchase at the Option of Holders — Change of Control."

Many of the covenants contained in the indenture will terminate if the notes are rated investment grade by either Standard & Poor's or Moody's and no default has occurred and is continuing.

Many of the covenants in the indenture governing the notes will terminate if the notes are rated investment grade by either Standard & Poor's or Moody's provided at such time no default or event of

S-9

Table of Contents

default has occurred and is continuing. The covenants will restrict, among other things, our ability to pay dividends, incur debt, and to enter into certain other transactions. There can be no assurance that the notes will ever be rated investment grade. However, termination of these covenants would allow us to engage in certain transactions that would not be permitted while these covenants were in force, and the effects of any such transactions will be permitted to remain in place even if the notes are subsequently downgraded below investment grade. Please read "Description of Notes — Covenants — Covenant Termination."

The subsidiary guarantees could be deemed fraudulent conveyances under certain circumstances, and a court may try to subordinate or void the subsidiary guarantees.

Under the federal bankruptcy laws and comparable provisions of state fraudulent transfer laws, a subsidiary guarantee could be voided, or claims in respect of a subsidiary guarantee could be subordinated to all other debts of that subsidiary guarantor if, among other things, the subsidiary guarantor, at the time it incurred the indebtedness evidenced by its subsidiary guarantee:

•: received less than reasonably equivalent value or fair consideration for the incurrence of such subsidiary guarantee; and
•: was insolvent or rendered insolvent by reason of such incurrence;
•: was engaged in a business or transaction for which the subsidiary guarantor's remaining assets constituted unreasonably small capital; or
•: intended to incur, or believed that it would incur, debts beyond its ability to pay such debts as they mature.

In addition, any payment by that subsidiary guarantor pursuant to its subsidiary guarantee could be voided and required to be returned to the subsidiary guarantor, or to a fund for the benefit of the creditors of the subsidiary guarantor. The measures of insolvency for purposes of these fraudulent transfer laws will vary depending upon the law applied in any proceeding to determine whether a fraudulent transfer has occurred. Generally, however, a subsidiary guarantor would be considered insolvent if:

•: the sum of its debts, including contingent liabilities, was greater than the fair saleable value of all of its assets;
•: the present fair saleable value of its assets was less than the amount that would be required to pay its probable liability, including contingent liabilities, on its existing debts, as they become absolute and mature; or
•: it could not pay its debts as they become due.

Your ability to sell the notes may be limited by the absence of a trading market.

The notes will not be listed on any securities exchange or included in any automated quotation system. The underwriters have advised us that, following completion of this offering, they intend to make a market in the notes as permitted by applicable law. The underwriters are not obligated, however, to make a market in the notes, and may discontinue any market making activities at any time without notice, in their sole discretion. If the underwriters fail to act as a market maker for the notes for any reason, there can be no assurance that another firm or person will make a market in the notes. Accordingly, we cannot assure you as to the development or liquidity of any market for the notes. Historically, the market for non-investment grade debt has been subject to disruptions that have caused substantial volatility in the prices of securities similar to the notes. We cannot assure you that the market, if any, for the notes will be free from similar disruptions. Any such disruption may adversely affect your ability to sell the notes.

S-10

Table of Contents

USE OF PROCEEDS

We estimate the net proceeds to us from this offering to be approximately $1,175.1 million, after deducting estimated fees and expenses payable by us (including underwriting discounts and commissions).

We intend to use the net proceeds from this offering to fund the payments we will make upon the settlement of our pending Tender Offers for any and all of our outstanding 2020 Notes, 2021 Notes and 2022 Notes. Please read "Summary — Tender Offers." Any amounts required to make the payments necessary at settlement of the Tender Offers that are not funded by the net proceeds from this offering will be funded partly by cash on hand, with the remainder to be funded through borrowings under the Credit Facility.

As of May 26, 2015, the outstanding balance under the Credit Facility, which matures in March 2019, was approximately $120.2 million (excluding approximately $11.3 million of outstanding letters of credit), and the weighted average interest rate of the Credit Facility was 3.6%. Existing borrowings under the Credit Facility were incurred to fund our ongoing capital expenditure program and for working capital purposes. Certain of the underwriters or their affiliates may be holders of notes subject to the Tender Offers and may therefore also receive a portion of the proceeds from this offering in respect of such notes. See "Underwriting."

S-11

Table of Contents

CAPITALIZATION

The following table sets forth our consolidated cash and cash equivalents and our consolidated capitalization as of March 31, 2015:

•: on an actual basis; and
•: on an as adjusted basis to give effect to the issuance of the notes in this offering and the application of the net proceeds therefrom in the manner described under "Use of Proceeds."

This table is derived from, should be read together with, and is qualified in its entirety by reference to the information contained in "Management's Discussion and Analysis of Financial Condition and Results of Operations" and our historical consolidated financial statements, including the related notes, contained in our quarterly report on Form 10-Q for the quarter ended March 31, 2015, which is incorporated by reference in this prospectus supplement.

	As of March 31, 2015

	Actual			As Adjusted
	(unaudited, in thousands)
Cash and cash equivalents(1)	$	236,284		$	175,284



Debt:
Credit Facility(2)	$	—		$	147,392
6.75% senior notes due 2020	$	500,000		$	—	(4)
6.5% senior notes due 2021	$	324,603	(3)	$	—	(4)
6.25% senior notes due 2022	$	455,000		$	—	(4)
5.5% senior notes due 2023	$	744,385	(3)	$	744,385	(3)
4.5% senior notes due 2023	$	1,000,000		$	1,000,000
4.875% senior notes due 2024	$	1,160,475	(3)	$	1,160,475	(3)
4.875% senior notes due 2025	$	—		$	1,188,312	(3)

Total debt	$	4,184,463		$	4,240,564
Equity:
Common units	$	4,584,275		$	4,472,990	(5)
Class B units	$	451,519		$	451,519
Non-controlling interest in consolidated subsidiaries	$	1,015,034		$	1,015,034

Total equity	$	6,050,828		$	5,939,543

Total capitalization	$	10,235,291		$	10,180,107

(1): Includes approximately $161.9 million of cash held by our non-guarantor subsidiaries, but does not give effect to the payment of approximately $23.0 million of accrued but unpaid interest on the 2020 Notes, 2021 Notes and 2022 Notes payable in connection with the Tender Offers based on the assumption in note (4) below.
(2): As of May 26, 2015, we had approximately $120.2 million of borrowings outstanding and $11.3 million of letters of credit outstanding under the Credit Facility.
(3): Net of unamortized discount and premium.
(4): As adjusted amount assumes all outstanding 2020 Notes, 2021 Notes and 2022 Notes are tendered prior to the expiration of the Tender Offers and purchased by us for cash at settlement on June 4, 2015 in accordance with the terms of the Tender Offers. However, this offering is not conditioned upon the consummation of any Tender Offer at any level of acceptance. Please read "Summary — Tender Offers."

S-12

Table of Contents

(5): Reflects decreases of approximately $118.5 million of loss on extinguishment of debt related to the Tender Offers, of which $14.6 million is related to the write-off of deferred financing costs and $0.4 million to the write-off of the unamortized bond discount on the 2021 Notes, based on the assumptions in note (4) above. These decreases are offset by a tax benefit of $7.2 million.

S-13

Table of Contents

RATIO OF EARNINGS TO FIXED CHARGES

The table below sets forth our ratios of earnings to fixed charges for each of the periods indicated. The ratio of earnings to fixed charges is calculated as earnings divided by fixed charges. Earnings consist of pre-tax income from continuing operations before fixed charges. Fixed charges consist of interest expense and capitalized interest, including the gain or loss on interest rate swaps and amortization of premiums, discounts and capitalized expenses related to indebtedness. Fixed charges also include an estimate of the interest within rental expense.

	Year Ended December 31,										Three Months Ended March 31, 2015

	2010		2011		2012		2013		2014		Three Months Ended March 31, 2015
Ratio of earnings to fixed charges		1.35x		2.00x		2.54x		1.15x		1.91x		1.30x

S-14

Table of Contents

DESCRIPTION OF NOTES

We will issue the notes under an indenture dated as of November 2, 2010, as supplemented by a supplemental indenture to be dated as of the Issue Date (collectively the "Indenture"), among us, the Subsidiary Guarantors and Wells Fargo Bank, National Association, as trustee (the "Trustee"). The terms of the notes include those expressly set forth in the Indenture and those made part of the Indenture by reference to the Trust Indenture Act of 1939, as amended (the "Trust Indenture Act"). The Indenture is unlimited in aggregate principal amount, although the issuance of notes in this offering will be limited to $1.2 billion.

You can find the definitions of certain terms in this description under the caption "— Definitions." In this description, the word "Issuers" refers only to MarkWest Energy Partners and MarkWest Finance and not to any of their subsidiaries and any reference to "MarkWest Energy Partners" or "MarkWest Finance" does not include any of their respective subsidiaries. As used in this section, "MarkWest Finance" means our subsidiary, MarkWest Energy Finance Corporation, which is a co-issuer of the notes.

This "Description of Notes," together with the "Description of Our Debt Securities" included in the accompanying base prospectus, is intended to be a useful overview of the material provisions of the notes and the Indenture. Since this "Description of Notes" and such "Description of Our Debt Securities" are only summaries, you should refer to the Indenture for a complete description of the obligations of the Issuers and your rights. This "Description of Notes" supersedes the "Description of Our Debt Securities" in the accompanying base prospectus to the extent it is inconsistent with such "Description of Our Debt Securities."

The registered holder of a note will be treated as the owner of it for all purposes. Only registered holders will have rights under the indenture.

Brief Description of the Notes and the Guarantees

The Notes

The notes will:

•: be general unsecured, senior obligations of the Issuers;
•: rank equally in right of payment to any existing and future senior Indebtedness of either of the Issuers, without giving effect to collateral arrangements, but are effectively subordinated to all present and future secured Indebtedness of either of the Issuers to the extent of the value of the collateral securing such Indebtedness; and
•: be unconditionally guaranteed on a senior, unsecured basis by the Subsidiary Guarantors.

The Guarantees

Initially, the notes will be guaranteed by MarkWest Energy Partners' principal operating company, MarkWest Energy Operating Company, L.L.C., which we refer to as the "Operating Company" in this description, and by all of MarkWest Energy Partners' other existing subsidiaries that guarantee both the Credit Facility and its outstanding series of senior notes.

Each Guarantee of a Subsidiary Guarantor of the notes will:

•: be a general unsecured, senior obligation of that Subsidiary Guarantor; and
•: rank equally in right of payment to any future senior Indebtedness of the Subsidiary Guarantor, without giving effect to collateral arrangements, but is effectively subordinated to all present and future secured Indebtedness of the Subsidiary Guarantor to the extent of the value of the collateral securing such Indebtedness.

S-15

Table of Contents

As a result of the effective subordination described above, in the event of a bankruptcy, liquidation or reorganization of MarkWest Energy Partners or MarkWest Finance, holders of these notes may recover less ratably than secured creditors of the Issuers who are holders of senior Indebtedness to the extent of the value of the collateral securing such Indebtedness.

As of March 31, 2015, on an as adjusted basis as described under "Capitalization," MarkWest Energy Partners (excluding its subsidiaries) would have had approximately $4.2 billion of total senior Indebtedness outstanding (excluding obligations under letters of credit and hedges), approximately $147.4 million of which would have been secured, and no Indebtedness contractually subordinated to the notes. On the same basis, the Subsidiary Guarantors would have had approximately $4.2 billion of senior Indebtedness outstanding (excluding intercompany debt and obligations under letters of credit and hedges), approximately $147.4 million of which would have been secured, and no Indebtedness contractually subordinated to their guarantees of the notes.

Currently, all of our Subsidiaries other than Liberty, MarkWest Utica EMG L.L.C., MarkWest Panola Pipeline L.L.C. and West Relay Gathering Company, L.L.C. and their respective Subsidiaries are "Restricted Subsidiaries." However, under the circumstances described below under the caption "— Covenants — Designation of Restricted and Unrestricted Subsidiaries," we may designate certain additional Restricted Subsidiaries as "Unrestricted Subsidiaries." Unrestricted Subsidiaries will not be subject to many of the restrictive covenants in the Indenture. Unrestricted Subsidiaries will not guarantee the notes.

MarkWest Energy Partners owns a 50% equity interest in MarkWest Pioneer, L.L.C. ("Pioneer"), the owner and operator of the Arkoma Connector Pipeline. In addition, MarkWest Energy Partners owns a 40% equity interest in Centrahoma Processing L.L.C. ("Centrahoma") that is accounted for using the equity method. Neither Pioneer, Centrahoma, nor any other unconsolidated subsidiary or equity investee will qualify as a "Subsidiary" under the Indenture, and, therefore, they will not be subject to any of the restrictive covenants in the Indenture nor will they guarantee the notes.

In the event of a bankruptcy, liquidation or reorganization of any of these non-guarantor consolidated subsidiaries or equity investees, they will pay the holders of their debt and their trade creditors before they will be able to distribute any of their assets to us.

For consolidated financial information as to our non-guarantor consolidated subsidiaries, please read footnote 15 in the notes to the condensed consolidated financial statements contained in our Quarterly Report on Form 10-Q for the quarter ended March 31, 2015 that is incorporated by reference herein.

Principal, Maturity and Interest

The Issuers will issue the notes in an aggregate principal amount of $1.2 billion. Subject to compliance with the covenant described below under the caption "— Covenants — Incurrence of Indebtedness and Issuance of Disqualified Equity," the Issuers may issue additional notes from time to time under the Indenture. The notes offered hereby and any additional notes subsequently issued under the Indenture will be treated as a single class for all purposes under the Indenture, including, without limitation, waivers, amendments, redemptions and offers to purchase. The Issuers may issue notes only in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof. The notes will mature on June 1, 2025.

Interest on the notes will accrue at the rate of 4.875% per annum and is payable semi-annually in arrears on June 1 and December 1, commencing on December 1, 2015. The Issuers will make each interest payment to the holders of record of the notes on the immediately preceding May 15 and November 15.

Interest on the notes will accrue from June 2, 2015. Interest will be computed on the basis of a 360-day year comprised of twelve 30-day months.

S-16

Table of Contents

Methods of Receiving Payments on the Notes

If a holder of more than $1.0 million has given wire transfer instructions to an account within the United States to the Issuers, the Issuers will make all payments of principal of, premium, if any, and interest on the notes in accordance with those instructions. All other payments on these notes will be made at the office or agency of the Paying Agent in Dallas, Texas, unless the Issuers elect to make interest payments by check mailed to the holders at their address set forth in the register of holders.

Paying Agent and Registrar for the Notes

The Trustee will act as Paying Agent and Registrar at its corporate trust office in Dallas, Texas. The Issuers may change the Paying Agent or Registrar without prior notice to the holders of the notes, and the Issuers or any of their Subsidiaries may act as Paying Agent or Registrar.

Transfer and Exchange

A holder may transfer or exchange notes in accordance with the Indenture. The Registrar and the Trustee may require a holder, among other things, to furnish appropriate endorsements and transfer documents and the Issuers may require a holder to pay any taxes and fees required by law or permitted by the Indenture. The Issuers are not required to transfer or exchange any note selected for redemption or repurchase (except in the case of a note to be redeemed or repurchased in part, the portion not to be redeemed or repurchased). Also, the Issuers are not required to transfer or exchange any note for a period of 15 days before a selection of notes to be redeemed or between a record date and the next succeeding interest payment date.

The registered holder of a note will be treated as the owner of it for all purposes, and all references in this description to "holders" are to holders of record.

The Guarantees

Initially, all of our wholly-owned Restricted Subsidiaries, excluding MarkWest Finance and MarkWest Energy West Texas Gas Company, L.L.C., will guarantee our Obligations under the notes and the Indenture. In the future, our Restricted Subsidiaries will be required to guarantee our Obligations under the notes and the Indenture in the circumstances described below under the caption "— Covenants — Additional Subsidiary Guarantees." We anticipate that MarkWest Energy West Texas Gas Company, L.L.C. will guarantee the notes shortly after their issuance.

The Subsidiary Guarantors will jointly and severally guarantee on a senior basis the Issuers' Obligations under the existing notes. The obligations of each Subsidiary Guarantor under its Guarantee rank equally in right of payment with other Indebtedness of such Subsidiary Guarantor, except to the extent such other Indebtedness is expressly subordinate to the obligations arising under the Guarantee. However, the notes will be structurally subordinated to the secured Indebtedness of the Subsidiary Guarantors to the extent of the value of the collateral securing such Indebtedness. The obligations of each Subsidiary Guarantor under its Guarantee will be limited as necessary to prevent that Guarantee from constituting a fraudulent conveyance under applicable law. See "Risk Factors."

As of March 31, 2015, on an as adjusted basis as described under "Capitalization," the Issuers and the Subsidiary Guarantors would have had approximately $4.2 billion of Indebtedness outstanding (excluding obligations under letters of credit and hedges and intercompany debt).

A Subsidiary Guarantor may not sell or otherwise dispose of all or substantially all of its properties or assets to, or consolidate with or merge with or into (whether or not such Subsidiary Guarantor is the surviving Person), another Person, except MarkWest Energy Partners or another Subsidiary Guarantor, unless:

(1) immediately after giving effect to that transaction, no Default or Event of Default exists; and

S-17

Table of Contents

(2) the Person acquiring the properties or assets in any such sale or other disposition or the Person formed by or surviving any such consolidation or merger (if other than the Subsidiary Guarantor) assumes all the Obligations of that Subsidiary Guarantor under its Guarantee and the Indenture pursuant to a supplemental indenture satisfactory to the Trustee, unless the Guarantee of such Subsidiary Guarantor is released as provided in clauses (1) and (2) of the next paragraph.

The Guarantee of a Subsidiary Guarantor will be released:

(1) in connection with any sale or other disposition of all or substantially all of the assets of that Subsidiary Guarantor (including by way of merger or consolidation) to a Person that is not (either before or after giving effect to such transaction) a Restricted Subsidiary, if the sale or other disposition does not violate the provisions of the Indenture applicable to Asset Sales;

(2) in connection with any sale or other disposition of the Equity Interests of a Subsidiary Guarantor to a Person that is not (either before or after giving effect to such transaction) an Issuer or a Restricted Subsidiary, if the sale or other disposition does not violate the provisions of the Indenture applicable to Asset Sales and the Subsidiary Guarantor ceases to be a Restricted Subsidiary of MarkWest Energy Partners as a result of the sale or other disposition;

(3) if MarkWest Energy Partners designates any Restricted Subsidiary that is a Subsidiary Guarantor as an Unrestricted Subsidiary;

(4) upon Legal Defeasance or Covenant Defeasance as described below under the caption "— Legal Defeasance and Covenant Defeasance" or upon satisfaction and discharge of the Indenture as described below under the caption "— Satisfaction and Discharge;"

(5) in the case of any Subsidiary Guarantor other than the Operating Company, at such time as such Subsidiary Guarantor ceases to guarantee any other Indebtedness of either of the Issuers and any Indebtedness of the Operating Company; or

(6) in the case of the Operating Company, at such time as the Operating Company ceases to guarantee any other Indebtedness of either of the Issuers, provided that it is then no longer an obligor with respect to any Indebtedness under any Credit Facility.

See "— Repurchase at the Option of Holders — Asset Sales."

Optional Redemption

The notes will be redeemable as a whole at any time or in part from time to time, at the option of the Issuers, at a redemption price equal to:

(x) if the redemption date is prior to March 1, 2025 (the date three months prior to the stated maturity of the notes), the greater of (i) 100% of the principal amount of the notes or (ii) the sum of the present values of the remaining scheduled payments of principal and interest thereon from the redemption date to March 1, 2025 (exclusive of any accrued interest) discounted to the redemption date on a semiannual basis (assuming a 360-day year consisting of twelve 30-day months) at the Treasury Rate plus 50 basis points; or

(y) if the redemption date is on or after March 1, 2025 (the date three months prior to the stated maturity of the notes), 100% of the principal amount of notes,

plus, in each case, any interest accrued but not paid to the date of redemption (subject to the right of holders of record on the relevant record date to receive interest due on an interest payment date that is on or prior to the redemption date).

"Treasury Rate" means, with respect to any redemption date, the rate per annum equal to the semiannual equivalent yield to maturity (computed by MarkWest Energy Partners as of the second business day immediately preceding such redemption date) of the Comparable Treasury Issue, assuming

S-18

Table of Contents

a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for such redemption date.

"Comparable Treasury Issue" means the United States Treasury security or securities selected by the Independent Investment Banker as having an actual or interpolated maturity comparable to the remaining term of the notes, calculated as if the maturity date of the notes were March 1, 2025 (the date three months prior to the stated maturity of the notes), that would be utilized, at the time of selection and in accordance with customary financial practice, in pricing new issues of corporate debt securities of a comparable maturity to the remaining term of the notes, calculated as if the maturity date of the notes were March 1, 2025 (the date three months prior to the stated maturity of the notes).

"Comparable Treasury Price" means, with respect to any redemption date, the average of the Reference Treasury Dealer Quotations for such redemption date.

"Independent Investment Banker" means one of the Reference Treasury Dealers appointed by MarkWest Energy Partners.

"Reference Treasury Dealer" means each of Wells Fargo Securities, LLC and a primary U.S. government securities dealer in the United States (a "Primary Treasury Dealer") selected by MarkWest Energy Partners, and their respective successors; provided, however, that if either of the foregoing shall not be a Primary Treasury Dealer at the appropriate time, MarkWest Energy Partners shall substitute therefor another Primary Treasury Dealer.

"Reference Treasury Dealer Quotations" means, with respect to each Reference Treasury Dealer and any redemption date, the average, as determined by the Independent Investment Banker, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to the Independent Investment Banker by such Reference Treasury Dealer at 3:30 p.m. New York time on the third business day preceding such redemption date.

Selection and Notice

If less than all of the notes are to be redeemed at any time, the Trustee will select notes for redemption as follows:

(1) if the notes are listed, in compliance with the requirements of the principal national securities exchange on which the notes are listed; or

(2) if the notes are not so listed or there are no such requirements, on a pro rata basis (or, in the case of notes in global form, the Trustee will select notes for redemption based on DTC's method that most nearly approximates a pro rata selection), by lot or by such method as the Trustee shall deem fair and appropriate.

No notes of $2,000 or less shall be redeemed in part. Notices of redemption shall be mailed by first class mail at least 30 but not more than 60 days before the redemption date to each holder of notes to be redeemed at its registered address, except that optional redemption notices may be mailed more than 60 days prior to a redemption date if the notice is issued in connection with a defeasance of the notes or a satisfaction and discharge of the Indenture.

Any such redemption may, at the discretion of MarkWest Energy Partners, be subject to one or more conditions precedent. If such redemption is subject to the satisfaction of one or more conditions precedent, the related notice shall describe each such condition, and if applicable, shall state that, in the discretion of MarkWest Energy Partners, the date of redemption may be delayed until such time as any or all such conditions shall be satisfied or waived (provided that in no event shall such date of redemption be delayed to a date later than 60 days after the date on which such notice was sent), or such redemption may not occur and such notice may be rescinded in the event that any or all such conditions shall not have been satisfied or waived by the date of redemption, or by the date of redemption as so delayed.

S-19

Table of Contents

If any note is to be redeemed in part only, the notice of redemption that relates to that note shall state the portion of the principal amount thereof to be redeemed. A new note in principal amount equal to the unredeemed portion of the original note will be issued in the name of the holder thereof upon cancellation of the original note. Notes called for redemption without condition become due on the date fixed for redemption. On and after the redemption date, interest ceases to accrue on notes or portions of them called for redemption unless the Issuers default in making such redemption payment.

Mandatory Redemption

Except as set forth below under "— Repurchase at the Option of Holders," neither of the Issuers is required to make mandatory redemption or sinking fund payments with respect to the notes or to repurchase the notes at the option of the Holders.

Repurchase at the Option of Holders

Change of Control

If a Change of Control occurs, each holder of notes will have the right to require MarkWest Energy Partners to repurchase all or any part (equal to $2,000 or an integral multiple of $1,000 in excess thereof) of that holder's notes pursuant to the Change of Control Offer. In the Change of Control Offer, MarkWest Energy Partners will offer a Change of Control Payment in cash equal to 101% of the aggregate principal amount of notes repurchased plus accrued and unpaid interest thereon, if any, to the date of purchase (the "Change of Control Payment Date"), subject to the rights of any holder in whose name a note is registered on a record date occurring prior to the Change of Control Payment Date to receive interest due on an interest payment date that is on or prior to such Change of Control Payment Date. Within 30 days following any Change of Control, MarkWest Energy Partners will mail a notice to each holder describing the transaction or transactions that constitute the Change of Control and offering to repurchase notes on the Change of Control Payment Date specified in such notice, pursuant to the procedures required by the Indenture and described in such notice. MarkWest Energy Partners will comply with the requirements of Rule 14e-1 under the Exchange Act and any other securities laws and regulations thereunder to the extent such laws and regulations are applicable in connection with the repurchase of the notes as a result of a Change of Control. To the extent that the provisions of any securities laws or regulations conflict with the Change of Control provisions of the Indenture, MarkWest Energy Partners will comply with the applicable securities laws and regulations and will not be deemed to have breached their obligations under the Change of Control provisions of the Indenture by virtue of such compliance.

On or before the Change of Control Payment Date, MarkWest Energy Partners will, to the extent lawful, accept for payment all notes or portions thereof properly tendered pursuant to the Change of Control Offer. Promptly after such acceptance, on the Change of Control Payment Date, MarkWest Energy Partners will:

(1) deposit with the Paying Agent an amount equal to the Change of Control Payment in respect of all notes or portions thereof so tendered; and

(2) deliver or cause to be delivered to the Trustee the notes so accepted together with an officers' certificate stating the aggregate principal amount of notes or portions thereof being purchased by MarkWest Energy Partners.

On the Change of Control Payment Date, the Paying Agent will mail to each holder of notes accepted for payment the Change of Control Payment for such notes (or, if all the notes are then in global form, make such payment through the facilities of DTC), and the Trustee will promptly authenticate and mail (or cause to be transferred by book entry) to each holder a new note equal in principal amount to any unpurchased portion of the notes surrendered, if any; provided that each such new note will be in a principal amount of $2,000 or an integral multiple of $1,000 in excess thereof. MarkWest

S-20

Table of Contents

Energy Partners will publicly announce the results of the Change of Control Offer on or as soon as practicable after the Change of Control Payment Date.

The provisions described above that require MarkWest Energy Partners to make a Change of Control Offer following a Change of Control will be applicable regardless of whether or not any other provisions of the Indenture are applicable. Except as described above with respect to a Change of Control, the Indenture will not contain provisions that permit the holder of the notes to require that the Issuers repurchase or redeem the notes in the event of a takeover, recapitalization or similar transaction.

MarkWest Energy Partners will not be required to make a Change of Control Offer upon a Change of Control if (1) a third party makes the Change of Control Offer in the manner, at the times and otherwise in compliance with the requirements set forth in the Indenture applicable to a Change of Control Offer made by MarkWest Energy Partners and purchases all notes properly tendered and not withdrawn under the Change of Control Offer, (2) notice of redemption of all outstanding notes has been given pursuant to the Indenture as described above under the caption "— Optional Redemption," unless and until there is a default in payment of the applicable redemption price or (3) in connection with or in contemplation of any Change of Control, MarkWest Energy Partners has made an offer to purchase (an "Alternate Offer") any and all notes validly tendered at a cash price equal to or higher than the Change of Control Payment and has purchased all notes properly tendered in accordance with the terms of such Alternate Offer.

A Change of Control Offer may be made in advance of a Change of Control, and conditioned upon the occurrence of the Change of Control, if a definitive agreement is in place for the Change of Control at the time of making the Change of Control Offer.

The Credit Agreement provides that certain change of control events with respect to MarkWest Energy Partners would constitute a default under the agreements governing such Indebtedness. Any future credit agreements or other agreements relating to Indebtedness to which MarkWest Energy Partners becomes a party may contain similar restrictions and provisions. Moreover, the exercise by the holders of their right to require MarkWest Energy Partners to repurchase the notes could cause a default under such Indebtedness, even if the Change of Control does not, due to the financial effect of such a repurchase on MarkWest Energy Partners. If a Change of Control occurs at a time when MarkWest Energy Partners is prohibited from purchasing notes, MarkWest Energy Partners could seek the consent of the lenders of the borrowings containing such prohibition to the purchase of notes or could attempt to refinance such borrowings. If MarkWest Energy Partners does not obtain such a consent or repay such borrowings, MarkWest Energy Partners will remain prohibited from purchasing notes. In such case, MarkWest Energy Partners' failure to purchase tendered notes would constitute an Event of Default under the Indenture, which would, in turn, in all likelihood constitute a default under such borrowings. Finally, MarkWest Energy Partners' ability to pay cash to the holders upon a repurchase may be limited by MarkWest Energy Partners' then existing financial resources. We cannot assure you that sufficient funds will be available when necessary to make any required repurchases.

The definition of Change of Control includes a phrase relating to the sale, lease, transfer, conveyance or other disposition of "all or substantially all" of the properties or assets of MarkWest Energy Partners and its Restricted Subsidiaries taken as a whole. Although there is a limited body of case law interpreting the phrase "substantially all," there is no precise established definition of the phrase under applicable law. Accordingly, the ability of a holder of notes to require MarkWest Energy Partners to repurchase such notes as a result of a sale, lease, transfer, conveyance or other disposition of less than all of the properties or assets of MarkWest Energy Partners and its Restricted Subsidiaries taken as a whole to another Person or group may be uncertain.

In the event that holders of not less than 90% of the aggregate principal amount of the outstanding notes accept a Change of Control Offer or an Alternate Offer and MarkWest Energy Partners (or a third party making the Change of Control Offer as provided above) purchases all of the notes held by such holders, the Issuers will have the right, upon not less than 30 nor more than 60 days' notice, given

S-21

Table of Contents

not more than 30 days following the purchase pursuant to the Change of Control Offer or Alternate Offer described above, as the case may be, to redeem all of the notes that remain outstanding following such purchase at a redemption price equal to the Change of Control Payment plus, to the extent not included in the Change of Control Payment, accrued and unpaid interest on the notes that remain outstanding, to the date of redemption (subject to the right of holders of record on the relevant record date to receive interest due on an interest payment date that is on or prior to the redemption date).

Asset Sales

MarkWest Energy Partners will not, and will not permit any of its Restricted Subsidiaries to, consummate an Asset Sale unless:

(1) MarkWest Energy Partners (or the Restricted Subsidiary, as the case may be) receives consideration at the time of such Asset Sale at least equal to the fair market value of the assets or Equity Interests issued or sold or otherwise disposed of;

(2) such fair market value is determined by (a) an executive officer of the General Partner if the value is less than $50.0 million, as evidenced by an officers' certificate delivered to the Trustee or (b) the Board of Directors of the General Partner if the value is $50.0 million or more, as evidenced by a resolution of such Board of Directors of the General Partner; and

(3) at least 75% of the aggregate consideration received by MarkWest Energy Partners and its Restricted Subsidiaries in the Asset Sale and all other Asset Sales since the Issue Date is in the form of cash or Cash Equivalents. For purposes of this provision, each of the following shall be deemed to be cash:

(a) any liabilities (as shown on MarkWest Energy Partners' most recent consolidated balance sheet) of MarkWest Energy Partners or any Restricted Subsidiary (other than contingent liabilities and liabilities that are by their terms subordinated to the notes or any Guarantee) that are assumed by the transferee of any such assets pursuant to a novation or indemnity agreement that releases MarkWest Energy Partners or such Restricted Subsidiary from, or indemnifies it against, further liability; and

(b) any securities, notes or other Obligations received by MarkWest Energy Partners or any such Restricted Subsidiary from such transferee that are within 120 days after the Asset Sale (subject to ordinary settlement periods) converted by such Issuer or such Restricted Subsidiary into cash (to the extent of the cash received in that conversion).

Within 360 days after the receipt of any Net Proceeds from an Asset Sale, MarkWest Energy Partners or a Restricted Subsidiary may apply (or enter into a definitive agreement for such application within such 360-day period, provided that such application occurs within 90 days after the end of such 360-day period) such Net Proceeds at its option:

(1) to repay senior Indebtedness of MarkWest Energy Partners and/or its Restricted Subsidiaries (or to make an offer to repurchase or redeem such Indebtedness, provided that such repurchase or redemption closes within 45 days after the end of such 360-day period);

(2) to make a capital expenditure in a Permitted Business;

(3) to acquire other long-term tangible assets that are used or useful in a Permitted Business; or

(4) to invest in any other Permitted Investments other than Investments in Cash Equivalents, Interest Swaps or Currency Agreements.

Pending the final application of any such Net Proceeds, MarkWest Energy Partners or a Restricted Subsidiary may temporarily reduce revolving credit borrowings or otherwise invest such Net Proceeds in any manner that is not prohibited by the Indenture.

S-22

Table of Contents

Any Net Proceeds from Asset Sales that are not applied or invested as provided in the second paragraph of this covenant will constitute Excess Proceeds. When the aggregate amount of Excess Proceeds exceeds $50.0 million, within ten days thereof MarkWest Energy Partners will make a pro rata offer (an "Asset Sale Offer") to all holders of notes and all holders of other Indebtedness that is pari passu with the notes containing provisions similar to those set forth in the Indenture with respect to offers to purchase or redeem with the proceeds of sales of assets to purchase or redeem the maximum principal amount of notes and such other pari passu Indebtedness that may be purchased or redeemed out of the Excess Proceeds. The offer price in any Asset Sale Offer will be equal to 100% of principal amount plus accrued and unpaid interest, if any, to the date of purchase, subject to the rights of any holder in whose name a note is registered on a record date occurring prior to the payment date to receive interest due on an interest payment date that is on or prior to such payment date, and will be payable in cash. If any Excess Proceeds remain after consummation of an Asset Sale Offer, MarkWest Energy Partners and its Restricted Subsidiaries may use such Excess Proceeds for any purpose not otherwise prohibited by the Indenture, including, without limitation, the repurchase or redemption of Indebtedness of the Issuers or any Subsidiary Guarantor that is subordinated to the notes or, in the case of any Subsidiary Guarantor, the Guarantee of such Subsidiary Guarantor. If the aggregate principal amount of notes tendered into such Asset Sale Offer exceeds the amount of Excess Proceeds allocated for repurchases of notes pursuant to the Asset Sale Offer for notes, the Trustee shall select the notes to be purchased on a pro rata basis (or, in the case of notes in global form, the Trustee will select notes for redemption based on DTC's method that most nearly approximates a pro rata selection). Upon completion of each Asset Sale Offer, the amount of Excess Proceeds shall be reset at zero.

MarkWest Energy Partners will comply with the requirements of Rule 14e-1 under the Exchange Act and any other securities laws and regulations thereunder to the extent those laws and regulations are applicable in connection with each repurchase of notes pursuant to an Asset Sale Offer. To the extent that the provisions of any securities laws or regulations conflict with the Asset Sale provisions of the Indenture, MarkWest Energy Partners will comply with the applicable securities laws and regulations and will not be deemed to have breached its obligations under the Asset Sale provisions of the Indenture by virtue of such compliance.

Covenants

Restricted Payments

MarkWest Energy Partners will not, and will not permit any of its Restricted Subsidiaries to, directly or indirectly:

(1) declare or pay any dividend or make any other payment or distribution on account of MarkWest Energy Partners' or any of its Restricted Subsidiaries' Equity Interests (including, without limitation, any payment in connection with any merger or consolidation involving MarkWest Energy Partners or any of its Restricted Subsidiaries) or to the direct or indirect holders of MarkWest Energy Partners' or any of its Restricted Subsidiaries' Equity Interests in their capacity as such (other than distributions or dividends payable in Equity Interests of MarkWest Energy Partners (other than Disqualified Equity) and other than distributions or dividends payable to MarkWest Energy Partners or a Restricted Subsidiary);

(2) purchase, redeem or otherwise acquire or retire for value (including, without limitation, in connection with any merger or consolidation involving an Issuer) any Equity Interests of MarkWest Energy Partners or any of its Restricted Subsidiaries (other than any such Equity Interests owned by MarkWest Energy Partners or any of its Restricted Subsidiaries);

(3) make any payment on or with respect to, or purchase, redeem, defease or otherwise acquire or retire for value any Subordinated Obligation or Guarantor Subordinated Obligation, except a payment of interest or principal at the Stated Maturity thereof or within one year of the Stated Maturity thereof; or

S-23

Table of Contents

(4) make any Investment other than a Permitted Investment (all such payments and other actions set forth in clauses (1) through (4) above being collectively referred to as "Restricted Payments")

unless, at the time of and after giving effect to such Restricted Payment, no Default (except a Reporting Default) or Event of Default shall have occurred and be continuing or would occur as a consequence thereof and either:

(1) if the Fixed Charge Coverage Ratio for MarkWest Energy Partners' four most recent fiscal quarters for which internal financial statements are available is not less than 1.75 to 1.0, such Restricted Payment, together with the aggregate amount of all other Restricted Payments made by MarkWest Energy Partners and its Restricted Subsidiaries during the quarter in which such Restricted Payment is made, is less than the sum, without duplication, of:

(a) Available Cash as of the end of the immediately preceding quarter, plus

(b) the sum of (i) the aggregate net cash proceeds of any (1) capital contribution to MarkWest Energy Partners from any Person (other than a Restricted Subsidiary of MarkWest Energy Partners) made after the 2012 Issue Date or (2) issuance and sale (other than to a Restricted Subsidiary of MarkWest Energy Partners) made after the Issue Date of Equity Interests (other than Disqualified Equity) of MarkWest Energy Partners or from the issuance or sale (other than to a Restricted Subsidiary of MarkWest Energy Partners) made after the 2012 Issue Date of convertible or exchangeable Disqualified Equity or convertible or exchangeable debt securities of MarkWest Energy Partners that have been converted into or exchanged for such Equity Interests (other than Disqualified Equity), and (ii) the fair market value of any Permitted Business or long-term tangible assets that are useful in a Permitted Business to the extent acquired in consideration of Equity Interests of MarkWest Energy Partners (other than Disqualified Equity) since the 2012 Issue Date, plus

(c) to the extent that any Restricted Investment that was made after the 2012 Issue Date is sold for cash or Cash Equivalents or otherwise liquidated or repaid for cash or Cash Equivalents, the lesser of the refund of capital or similar payment made in cash or Cash Equivalents with respect to such Restricted Investment (less the cost of such disposition, if any) and the initial amount of such Restricted Investment (other than to a Restricted Subsidiary of MarkWest Energy Partners), plus

(d) the net reduction in Restricted Investments resulting from dividends, repayments of loans or advances, or other transfers of assets in each case to MarkWest Energy Partners or any of its Restricted Subsidiaries from any Person (including, without limitation, Unrestricted Subsidiaries) or from redesignations of Unrestricted Subsidiaries as Restricted Subsidiaries, to the extent such amounts have not been included in Available Cash for any period commencing on or after the 2012 Issue Date (items (b), (c) and (d) being referred to as "Incremental Funds"), minus

(e) the aggregate amount of Incremental Funds previously expended pursuant to this clause (1) or clause (2) below; or

(2) if the Fixed Charge Coverage Ratio for MarkWest Energy Partners' four most recent fiscal quarters for which internal financial statements are available is less than 1.75 to 1.0, such Restricted Payment together with the aggregate amount of all other Restricted Payments made by MarkWest Energy Partners and its Restricted Subsidiaries during the quarter in which such Restricted Payment is made (such Restricted Payments for purposes of this clause (2) meaning

S-24

Table of Contents

only distributions on common units of MarkWest Energy Partners, plus the related distribution on the general partner interest) is less than the sum, without duplication, of:

(a) $200.0 million less the aggregate amount of all prior Restricted Payments made by MarkWest Energy Partners and its Restricted Subsidiaries pursuant to this clause (2)(a) since the 2012 Issue Date, plus

(b) Incremental Funds to the extent not previously expended pursuant to this clause (2) or clause (1) above.

The preceding provisions will not prohibit:

(1) the payment by MarkWest Energy Partners or any Restricted Subsidiary of any distribution or dividend within 60 days after the date of declaration thereof, if at said date of declaration such payment would have complied with the provisions of the Indenture;

(2) the redemption, repurchase, retirement, defeasance or other acquisition of any Subordinated Obligation or any Guarantor Subordinated Obligation or of any Equity Interests of MarkWest Energy Partners in exchange for, or out of the net cash proceeds of, a substantially concurrent (a) capital contribution to MarkWest Energy Partners from any Person (other than a Restricted Subsidiary of MarkWest Partners) or (b) sale (other than to a Restricted Subsidiary of MarkWest Energy Partners) of Equity Interests (other than Disqualified Equity) of MarkWest Energy Partners, with a sale being deemed substantially concurrent if such redemption, repurchase, retirement, defeasance or other acquisition occurs not more than 120 days after such sale; provided that the amount of any such net cash proceeds that are utilized for any such redemption, repurchase, retirement, defeasance or other acquisition will be excluded or deducted from the calculation of Available Cash and Incremental Funds;

(3) the redemption, repurchase, retirement, defeasance or other acquisition of any Subordinated Obligation or Guarantor Subordinated Obligation with the net cash proceeds from an incurrence of, or in exchange for, Permitted Refinancing Indebtedness;

(4) the payment of any distribution or dividend by a Restricted Subsidiary to MarkWest Energy Partners or to the holders of its Equity Interests (other than Disqualified Equity) on a pro rata basis; and

(5) so long as no Default (except a Reporting Default) has occurred and is continuing or would be caused thereby, the repurchase, redemption or other acquisition or retirement for value of any Equity Interests of MarkWest Energy Partners or any Restricted Subsidiary of MarkWest Energy Partners pursuant to any management equity subscription agreement or equity option agreement or other employee benefit plan or to satisfy obligations under any Equity Interests appreciation rights or option plan or similar arrangement; provided that the aggregate price paid for all such repurchased, redeemed, acquired or retired Equity Interests shall not exceed $3.0 million in any calendar year.

In computing the amount of Restricted Payments previously made for purposes of the first paragraph of this section, Restricted Payments made under clauses (1) (but only if the declaration of such dividend or other distribution has not been counted in a prior period) and, to the extent of amounts paid to holders other than MarkWest Energy Partners or a Restricted Subsidiary, (4) of this paragraph shall be included, and Restricted Payments made under clauses (2), (3) and (5) and, except to the extent noted above, (4) of this paragraph shall not be included. The amount of all Restricted Payments (other than cash) shall be the fair market value, on the date of the Restricted Payment, of the Restricted Investment proposed to be made or the asset(s) or securities proposed to be transferred or issued by MarkWest Energy Partners or such Restricted Subsidiary, as the case may be, pursuant to the Restricted Payment, except that the fair market value of any non-cash distribution or dividend paid within 60 days after the date of declaration will be determined as of such date of declaration.

S-25

Table of Contents

Incurrence of Indebtedness and Issuance of Disqualified Equity

MarkWest Energy Partners will not, and will not permit any of its Restricted Subsidiaries to, directly or indirectly, create, incur, issue, assume, guarantee or otherwise become directly or indirectly liable, contingently or otherwise, with respect to (collectively, "incur") any Indebtedness (including Acquired Debt), and MarkWest Energy Partners will not issue any Disqualified Equity and will not permit any of its Restricted Subsidiaries to issue any Disqualified Equity; provided, however, that MarkWest Energy Partners and any Restricted Subsidiary may incur Indebtedness (including Acquired Debt), and MarkWest Energy Partners and the Restricted Subsidiaries may issue Disqualified Equity, if the Fixed Charge Coverage Ratio for MarkWest Energy Partners' most recently ended four full fiscal quarters for which internal financial statements are available immediately preceding the date on which such additional Indebtedness is incurred or such Disqualified Equity is issued would have been at least 2.0 to 1.0, determined on a pro forma basis (including a pro forma application of the net proceeds therefrom), as if the additional Indebtedness had been incurred, or the Disqualified Equity had been issued, as the case may be, at the beginning of such four-quarter period.

The first paragraph of this covenant will not prohibit the incurrence of any of the following items of Indebtedness (collectively, "Permitted Debt"):

(1) the incurrence by MarkWest Energy Partners and any Restricted Subsidiary of Indebtedness under Credit Facilities and the guarantees thereof; provided that the aggregate principal amount of all Indebtedness of MarkWest Energy Partners and the Restricted Subsidiaries incurred pursuant to this clause (1) and outstanding under all Credit Facilities after giving effect to such incurrence does not exceed the greater of (a) $1.8 billion or (b) $500.0 million plus 25.0% of the Consolidated Net Tangible Assets of MarkWest Energy Partners;

(2) the incurrence by MarkWest Energy Partners and its Restricted Subsidiaries of Existing Indebtedness (other than under the Credit Facilities);

(3) the incurrence by the Issuers and the Subsidiary Guarantors of Indebtedness represented by the notes issued and sold in this offering and the related Guarantees;

(4) the incurrence by MarkWest Energy Partners or any of its Restricted Subsidiaries of Indebtedness represented by Capital Lease Obligations, mortgage financings or purchase money obligations, in each case, incurred for the purpose of financing all or any part of the purchase price or cost of construction or improvement of property, plant or equipment used in the business of MarkWest Energy Partners or such Restricted Subsidiary, including all Permitted Refinancing Indebtedness incurred to extend, refinance, renew, replace, defease or refund any Indebtedness incurred pursuant to this clause (4), provided that after giving effect to any such incurrence, the principal amount of all Indebtedness incurred pursuant to this clause (4) and then outstanding does not exceed the greater of (a) $125.0 million or (b) 2.5% of the Consolidated Net Tangible Assets of MarkWest Energy Partners at such time;

(5) the incurrence by MarkWest Energy Partners or any of its Restricted Subsidiaries of Permitted Refinancing Indebtedness in exchange for, or the net proceeds of which are used to extend, refinance, renew, replace, defease or refund, Indebtedness that was permitted by the Indenture to be incurred under the first paragraph of this covenant or clause (2) or (3) of this paragraph or this clause (5);

(6) the incurrence by MarkWest Energy Partners or any of its Restricted Subsidiaries of intercompany Indebtedness between or among MarkWest Energy Partners and any of its Restricted Subsidiaries; provided, however, that:

(a) if MarkWest Energy Partners is the obligor on such Indebtedness and a Subsidiary Guarantor is not the obligee, such Indebtedness must be expressly subordinated to the prior payment in full in cash of all Obligations with respect to the notes, or if a Subsidiary

S-26

Table of Contents

Guarantor is the obligor on such Indebtedness and neither MarkWest Energy Partners nor another Subsidiary Guarantor is the obligee, such Indebtedness must be expressly subordinated to the prior payment in full in cash of all Obligations with respect to the Guarantee of such Subsidiary Guarantor; and

(b) (i) any subsequent issuance or transfer of Equity Interests that results in any such Indebtedness being held by a Person other than MarkWest Energy Partners or a Restricted Subsidiary thereof and (ii) any sale or other transfer of any such Indebtedness to a Person that is neither MarkWest Energy Partners nor a Restricted Subsidiary thereof, shall be deemed, in each case, to constitute an incurrence of such Indebtedness by MarkWest Energy Partners or such Restricted Subsidiary, as the case may be, that was not permitted by this clause (6);

(7) the incurrence by MarkWest Energy Partners or any of its Restricted Subsidiaries of Hedging Obligations that are incurred for the purpose of fixing or hedging (but not for speculative purposes) (a) foreign currency exchange rate risks of MarkWest Energy Partners or any Restricted Subsidiary, (b) interest rate risks with respect to any floating rate Indebtedness of MarkWest Energy Partners or any Restricted Subsidiary that is permitted by the terms of the Indenture to be outstanding or (c) commodities pricing risks of MarkWest Energy Partners or any Restricted Subsidiary in respect of Hydrocarbons used, produced, processed or sold by MarkWest Energy Partners or any of its Restricted Subsidiaries;

(8) the guarantee by MarkWest Energy Partners or any of its Restricted Subsidiaries of Indebtedness of MarkWest Energy Partners or any of its Restricted Subsidiaries that was permitted to be incurred by another provision of this covenant, provided, that in the event such Indebtedness that is being guaranteed is a Subordinated Obligation or a Guarantor Subordinated Obligation, then the guarantee shall be subordinated in right of payment to the notes or the Guarantee, as the case may be;

(9) bid, performance, surety and appeal bonds incurred in the ordinary course of business, including guarantees and obligations respecting standby letters of credit supporting such obligations, to the extent not drawn (in each case other than an obligation for money borrowed);

(10) the incurrence by MarkWest Energy Partners or any of its Restricted Subsidiaries of liability in respect of the Indebtedness of any Unrestricted Subsidiary of MarkWest Energy Partners or any Joint Venture but only to the extent that such liability is the result of MarkWest Energy Partners' or any such Restricted Subsidiary's being a general partner of such Unrestricted Subsidiary or Joint Venture and not as guarantor of such Indebtedness and provided that, after giving effect to any such incurrence, the aggregate principal amount of all Indebtedness incurred under this clause (10) and then outstanding does not exceed $25.0 million;

(11) the incurrence by MarkWest Energy Partners or any of its Restricted Subsidiaries of Acquired Debt in connection with a merger or consolidation meeting either one of the financial tests set forth in clause (4) under the caption "— Merger, Consolidation or Sale of Assets;" and

(12) the incurrence by MarkWest Energy Partners or any of its Restricted Subsidiaries of additional indebtedness in an aggregate principal amount at any time outstanding not to exceed the greater of (a) $250.0 million or (b) 5.0% of the Consolidated Net Tangible Assets of MarkWest Energy Partners.

For purposes of determining compliance with this "— Incurrence of Indebtedness and Issuance of Disqualified Equity" covenant, in the event that an item of proposed Indebtedness meets the criteria of more than one of the categories of Permitted Debt described in clauses (1) through (12) above, or is entitled to be incurred pursuant to the first paragraph of this covenant, MarkWest Energy Partners will be permitted to classify (or later reclassify in whole or in part) such item of Indebtedness in any manner that complies with this covenant. An item of Indebtedness may be divided and classified in one

S-27

Table of Contents

or more of the types of Permitted Indebtedness. Any Indebtedness under Credit Facilities on the Issue Date shall be considered incurred under the first paragraph of this covenant.

The accrual of interest, the accretion or amortization of original issue discount, the payment of interest on any Indebtedness in the form of additional indebtedness with the same terms, and the payment of dividends on Disqualified Equity in the form of additional shares of the same class of Disqualified Equity will not be deemed to be an incurrence of Indebtedness or an issuance of Disqualified Equity for purposes of this covenant; provided, in each such case, that the amount thereof is included in Fixed Charges of MarkWest Energy Partners as accrued. Notwithstanding any other provision of this covenant, the maximum amount of Indebtedness that MarkWest Energy Partners or any Restricted Subsidiary may incur pursuant to this covenant shall not be deemed to be exceeded solely as a result of fluctuations in exchange rates or currency values. Further, the accounting reclassification of any obligation of MarkWest Energy Partners or any of its Restricted Subsidiaries as Indebtedness will not be deemed an incurrence of Indebtedness for purposes of this covenant.

Liens

MarkWest Energy Partners will not, and will not permit any of its Restricted Subsidiaries to create, incur, assume or suffer to exist any Lien of any kind securing Indebtedness upon any asset now owned or hereafter acquired, except Permitted Liens, without making effective provision whereby all Obligations due under the notes and Indenture or any Guarantee, as applicable, will be secured by a Lien equally and ratably with (or prior to in the case of Liens with respect to Subordinated Obligations or Guarantor Subordinated Obligations, as the case may be) any and all Obligations thereby secured for so long as any such Obligations shall be so secured.

Dividend and Other Payment Restrictions Affecting Subsidiaries

MarkWest Energy Partners will not, and will not permit any of its Restricted Subsidiaries to, directly or indirectly, create or permit to exist or become effective any encumbrance or restriction on the ability of any Restricted Subsidiary to:

(1) pay dividends or make any other distributions on its Equity Interests to MarkWest Energy Partners or any of MarkWest Energy Partners' Restricted Subsidiaries, or pay any indebtedness or other obligations owed to MarkWest Energy Partners or any of the other Restricted Subsidiaries;

(2) make loans or advances to or make other investments in MarkWest Energy Partners or any of the other Restricted Subsidiaries; or

(3) transfer any of its properties or assets to MarkWest Energy Partners or any of the other Restricted Subsidiaries.

However, the preceding restrictions will not apply to encumbrances or restrictions existing under or by reason of:

(1) agreements as in effect on the Issue Date and any amendments, modifications, restatements, renewals, increases, supplements, refundings, replacements or refinancings of any such agreements or any Existing Indebtedness to which such agreement relates, provided that such amendments, modifications, restatements, renewals, increases, supplements, refundings, replacements or refinancings are no more restrictive, taken as a whole, with respect to such distribution, dividend and other payment restrictions and loan or investment restrictions than those contained in such agreement, as in effect on the Issue Date;

(2) the Indenture, the notes and the Guarantees;

(3) applicable law;

S-28

Table of Contents

(4) any instrument governing Indebtedness or Equity Interests of a Person acquired by MarkWest Energy Partners or any of its Restricted Subsidiaries as in effect at the time of such acquisition (except to the extent such Indebtedness was incurred in connection with or in contemplation of such acquisition), which encumbrance or restriction is not applicable to any Person, or the property or assets of any Person, other than such Person, or the property or assets of such Person, so acquired, provided that, in the case of Indebtedness, such Indebtedness was permitted by the terms of the Indenture to be incurred;

(5) customary non-assignment provisions in agreements for the purchase, gathering, processing, sale, transportation or exchange of Hydrocarbons or similar operational agreements or in licenses, leases, rights-of-way or other easements or servitudes, in each case entered into in the ordinary course of business;

(6) Capital Lease Obligations, mortgage financings or purchase money obligations, in each case for property acquired in the ordinary course of business that impose restrictions on that property of the nature described in clause (3) of the preceding paragraph;

(7) any agreement for the sale or other disposition of a Restricted Subsidiary that restricts distributions by that Restricted Subsidiary pending its sale or other disposition;

(8) Permitted Refinancing Indebtedness, provided that the restrictions contained in the agreements governing such Permitted Refinancing Indebtedness are no more restrictive, taken as a whole, than those contained in the agreements governing the Indebtedness being refinanced;

(9) Liens securing Indebtedness otherwise permitted to be incurred pursuant to the provisions of the covenant described above under the caption "— Liens" that limit the right of MarkWest Energy Partners or any of its Restricted Subsidiaries to dispose of the assets subject to such Lien;

(10) provisions with respect to the disposition or distribution of assets or property in joint venture agreements, asset sale agreements, stock sale agreements and other similar agreements entered into in the ordinary course of business;

(11) any agreement or instrument relating to any property or assets acquired after the Issue Date, so long as such encumbrance or restriction relates only to the property or assets so acquired and is not and was not created in anticipation of such acquisitions;

(12) restrictions on cash or other deposits or net worth imposed by customers under contracts entered into in the ordinary course of business;

(13) with respect to any Foreign Subsidiary, any encumbrance or restriction contained in the terms of any Indebtedness or any agreement pursuant to which such Indebtedness was incurred if either (a) the encumbrance or restriction applies only in the event of a payment default or a default with respect to a financial covenant in such Indebtedness or agreement or (b) MarkWest Energy Partners determines that any such encumbrance or restriction will not materially affect the Issuers' ability to make principal or interest payments on the notes, as determined in good faith by the Board of Directors of the General Partner, whose determination shall be conclusive; and

(14) any other agreement governing Indebtedness of MarkWest Energy Partners or any Restricted Subsidiary that is permitted to be incurred by the covenant described under "— Incurrence of Indebtedness and Issuance of Disqualified Equity"; provided, however, that such encumbrances or restrictions are not materially more restrictive, taken as a whole, than those contained in the Indenture or the Credit Agreement as it exists on the Issue Date.

Merger, Consolidation or Sale of Assets

Neither of the Issuers may, directly or indirectly: (1) consolidate or merge with or into another Person (whether or not such Issuer is the survivor); or (2) sell, assign, transfer, lease, convey or otherwise

S-29

Table of Contents

dispose of all or substantially all of its properties or assets, in one or more related transactions, to another Person; unless:

(1) either: (a) such Issuer is the surviving entity of such transaction; or (b) the Person formed by or surviving any such consolidation or merger (if other than such Issuer) or to which such sale, assignment, transfer, lease, conveyance or other disposition shall have been made is an entity organized or existing under the laws of the United States, any state thereof or the District of Columbia, provided that MarkWest Finance may not consolidate or merge with or into any entity other than a corporation satisfying such requirement for so long as MarkWest Energy Partners is not a corporation;

(2) the Person formed by or surviving any such consolidation or merger (if other than such Issuer) or the Person to which such sale, assignment, transfer, lease, conveyance or other disposition shall have been made expressly assumes all the Obligations of such Issuer under the notes and the Indenture pursuant to agreements reasonably satisfactory to the Trustee;

(3) immediately after such transaction no Default or Event of Default exists;

(4) in the case of a transaction involving MarkWest Energy Partners and not MarkWest Finance, either

(a) MarkWest Energy Partners or the Person formed by or surviving any such consolidation or merger (if other than MarkWest Energy Partners), or to which such sale, assignment, transfer, lease, conveyance or other disposition has been made will, on the date of such transaction after giving pro forma effect thereto and any related financing transactions as if the same had occurred at the beginning of the applicable four-quarter period, be permitted to incur at least $1.00 of additional Indebtedness pursuant to the Fixed Charge Coverage Ratio test set forth in the first paragraph of the covenant described above under the caption "— Incurrence of Indebtedness and Issuance of Disqualified Equity;" or

(b) immediately after giving effect to such transaction on a pro forma basis and any related financing transactions as if the same had occurred at the beginning of the applicable four-quarter period, the Fixed Charge Coverage Ratio of MarkWest Energy Partners or the Person formed by or surviving any such consolidation or merger (if other than MarkWest Partners) or to which such sale, assignment, transfer, lease, conveyance or other disposition has been made, will be equal to or greater than the Fixed Charge Coverage Ratio of MarkWest Energy Partners immediately before such transactions; and

(5) such Issuer has delivered to the Trustee an officers' certificate and an opinion of counsel, each stating that such consolidation, merger or disposition and, if a supplemental indenture is required, such supplemental indenture comply with the Indenture and all conditions precedent therein relating to such transaction have been satisfied.

Notwithstanding the preceding paragraph, MarkWest Energy Partners is permitted to reorganize as any other form of entity in accordance with the procedures established in the Indenture; provided that:

(1) the reorganization involves the conversion (by merger, sale, contribution or exchange of assets or otherwise) of MarkWest Energy Partners into a form of entity other than a limited partnership formed under Delaware law;

(2) the entity so formed by or resulting from such reorganization is an entity organized or existing under the laws of the United States, any state thereof or the District of Columbia;

(3) the entity so formed by or resulting from such reorganization assumes all the Obligations of MarkWest Energy Partners under the notes and the Indenture pursuant to agreements reasonably satisfactory to the Trustee;

(4) immediately after such reorganization no Default or Event of Default exists; and

S-30

Table of Contents

(5) such reorganization is not adverse to the holders of the notes (for purposes of this clause (5) it is stipulated that such reorganization shall not be considered adverse to the holders of the notes solely because the successor or survivor of such reorganization (a) is subject to federal or state income taxation as an entity or (b) is considered to be an "includible corporation" of an affiliated group of corporations within the meaning of Section 1504(b) of the Code or any similar state or local law).

Although there is a limited body of case law interpreting the phrase "substantially all," there is no precise established definition of the phrase under applicable law. Accordingly, in certain circumstances there may be a degree of uncertainty as to whether a particular transaction would involve "all or substantially all" of the properties or assets of a Person.

Transactions with Affiliates

MarkWest Energy Partners will not, and will not permit any of its Restricted Subsidiaries to, make any payment to, or sell, lease, transfer or otherwise dispose of any of its properties or assets to, or purchase any property or assets from, or enter into or make or amend any transaction, contract, agreement, understanding, loan, advance or guarantee with, or for the benefit of, any Affiliate (each, an "Affiliate Transaction"), unless:

(1) such Affiliate Transaction is on terms that are no less favorable to MarkWest Energy Partners or the relevant Restricted Subsidiary than those that would have been obtained in a comparable transaction by MarkWest Energy Partners or such Restricted Subsidiary with an unrelated Person; and

(2) MarkWest Energy Partners delivers to the Trustee, with respect to any Affiliate Transaction or series of related Affiliate Transactions involving aggregate consideration in excess of $50.0 million, a resolution of the Board of Directors of the General Partner set forth in an officers' certificate certifying that such Affiliate Transaction complies with this covenant and that such Affiliate Transaction has been approved by a majority of the disinterested members of the Board of Directors of the General Partner.

The following items shall not be deemed to be Affiliate Transactions and, therefore, will not be subject to the provisions of the prior paragraph:

(1) any employment, equity option or equity appreciation agreement or plan entered into by MarkWest Energy Partners or any of its Restricted Subsidiaries in the ordinary course of business;

(2) transactions between or among MarkWest Energy Partners and/or its Restricted Subsidiaries;

(3) Permitted Investments or Restricted Payments that are permitted by the provisions of the Indenture described above under the caption "— Restricted Payments;"

(4) customary compensation, indemnification and other benefits made available to officers, directors or employees of MarkWest Energy Partners or a Restricted Subsidiary, including reimbursement or advancement of out-of -pocket expenses and provisions of officers' and directors' liability insurance;

(5) sales of Equity Interests (other than Disqualified Equity) to Affiliates of MarkWest Energy Partners;

(6) in the case of contracts for gathering, transporting, treating, processing, marketing, distributing, storing or otherwise handling Hydrocarbons, or activities or services reasonably related or ancillary thereto, or other operational contracts, any such contracts are entered into in the

S-31

Table of Contents

ordinary course of business on terms substantially similar to those contained in similar contracts entered into by MarkWest Energy Partners or any Restricted Subsidiary and third parties;

(7) transactions with a Person (other than an Unrestricted Subsidiary of MarkWest Energy Partners) that is an Affiliate of MarkWest Energy Partners solely because MarkWest Energy Partners owns an Equity Interest in such Person;

(8) transactions between MarkWest Energy Partners or any of its Restricted Subsidiaries and any Person that would not otherwise constitute an Affiliate Transaction except for the fact that one director of such other Person is also a director of the General Partner or such Restricted Subsidiary, as applicable; provided that such director abstains from voting as a director of the General Partner or such Restricted Subsidiary, as applicable, on any matter involving such other Person; and

(9) transactions involving an Affiliate of MarkWest Energy Partners in which an unrelated third Person owns Voting Stock at least equal to that owned by MarkWest Energy Partners or any of its Restricted Subsidiaries.

Additional Subsidiary Guarantees

If, after the Issue Date, any Restricted Subsidiary of MarkWest Energy Partners that is not already a Subsidiary Guarantor guarantees any other Indebtedness of either of the Issuers or any Indebtedness of the Operating Company, or if the Operating Company, if not then a Subsidiary Guarantor, guarantees any other Indebtedness of either of the Issuers or incurs any Indebtedness under any Credit Facility, then in either case such Subsidiary must become a Subsidiary Guarantor by executing a supplemental indenture in the form specified in the Indenture and delivering an opinion of counsel to the Trustee within 20 business days of the date on which it guaranteed or incurred such Indebtedness. Notwithstanding the preceding, any Guarantee of a Restricted Subsidiary that was incurred pursuant to this paragraph shall be released in the circumstances described under the caption "— The Guarantees."

Designation of Restricted and Unrestricted Subsidiaries

The Board of Directors of the General Partner may designate any Restricted Subsidiary to be an Unrestricted Subsidiary if that designation would not cause a Default or Event of Default. If a Restricted Subsidiary is designated as an Unrestricted Subsidiary, all outstanding Investments owned by MarkWest Energy Partners and its Restricted Subsidiaries in the Subsidiary so designated will be deemed to be an Investment made as of the time of such designation and will either reduce the amount available for Restricted Payments under the first paragraph of the covenant described above under the caption "— Restricted Payments," or represent Permitted Investments. All such outstanding Investments will be valued at their fair market value at the time of such designation. That designation will only be permitted if such Restricted Payment or Permitted Investments would be permitted at that time and such Restricted Subsidiary otherwise meets the definition of an Unrestricted Subsidiary. All Subsidiaries of an Unrestricted Subsidiary shall also automatically be Unrestricted Subsidiaries. Upon the designation of a Restricted Subsidiary that is a Subsidiary Guarantor as an Unrestricted Subsidiary, the Guarantee of such entity shall be released.

The Board of Directors of the General Partner may at any time designate any Unrestricted Subsidiary to be a Restricted Subsidiary; provided that such designation shall be deemed to be an incurrence of Indebtedness by a Restricted Subsidiary of MarkWest Energy Partners of any outstanding Indebtedness of such Unrestricted Subsidiary and such designation shall only be permitted if (1) such Indebtedness is permitted under the covenant described under the caption "— Incurrence of Indebtedness and Issuance of Disqualified Equity," calculated on a pro forma basis as if such designation had occurred at the beginning of the four-quarter reference period and (2) no Default or Event of Default would be in existence following such designation.

S-32

Table of Contents

Business Activities of MarkWest Finance

MarkWest Finance may not incur Indebtedness unless (1) MarkWest Energy Partners is a co-obligor or guarantor of such Indebtedness or (2) the net proceeds of such Indebtedness are loaned to MarkWest Energy Partners, used to acquire outstanding debt securities issued by MarkWest Energy Partners or used to repay Indebtedness of MarkWest Energy Partners as permitted under the covenant described above under the caption "— Incurrence of Indebtedness and Issuance of Disqualified Equity." MarkWest Finance may not engage in any business not related directly or indirectly to obtaining money or arranging financing for MarkWest Energy Partners or its Restricted Subsidiaries.

Reports

Whether or not required by the SEC, so long as any notes are outstanding, MarkWest Energy Partners will file with the SEC (unless the SEC will not accept such a filing) within the time periods specified in the SEC's rules and regulations, and upon request, MarkWest Energy Partners will furnish (without exhibits) to the Trustee for delivery to the holders of the notes:

(1) all quarterly and annual financial information that would be required to be contained in a filing with the SEC on Forms 10-Q and 10-K if MarkWest Energy Partners were required to file such forms, including a "Management's Discussion and Analysis of Financial Condition and Results of Operations" and, with respect to the annual information only, a report on the annual financial statements by MarkWest Energy Partners' certified independent accountants; and

(2) all current reports that would be required to be filed with the SEC on Form 8-K if MarkWest Energy Partners were required to file such reports.

The availability of the foregoing information or reports on the SEC's website will be deemed to satisfy the foregoing delivery requirements.

If as of the end of any such quarterly or annual period MarkWest Energy Partners has designated any of its Subsidiaries as Unrestricted Subsidiaries, then MarkWest Energy Partners shall deliver (promptly after such SEC filing referred to in the preceding paragraph) to the Trustee for delivery to the holders of the notes quarterly and annual financial information required by the preceding paragraph as revised to include a reasonably detailed presentation, either on the face of the financial statements or in the footnotes thereto, and in "Management's Discussion and Analysis of Financial Condition and Results of Operations," of the financial condition and results of operations of MarkWest Energy Partners and its Restricted Subsidiaries separate from the financial condition and results of operations of the Unrestricted Subsidiaries of MarkWest Energy Partners.

In addition, whether or not required by the SEC, MarkWest Energy Partners will make such information available to securities analysts, investors and prospective investors upon request.

Any and all Defaults or Events of Default arising from a failure to furnish or file in a timely manner any information or report required by this covenant shall be deemed cured (and MarkWest Energy Partners shall be deemed to be in compliance with this covenant) upon furnishing or filing such information or report as contemplated by this covenant (but without regard to the date on which such information or report is so furnished or filed); provided that such cure shall not otherwise affect the rights of the holders under "— Events of Defaults and Remedies" if the principal, premium, if any, and interest have been accelerated in accordance with the terms of the Indenture and such acceleration has not been rescinded or cancelled prior to such cure.

Covenant Termination

If at any time (a) the notes have an Investment Grade Rating from either of the Rating Agencies, (b) no Default has occurred and is continuing under the Indenture, and (c) the Issuers have delivered to the Trustee an officers' certificate certifying to the foregoing provisions of this sentence, MarkWest

S-33

Table of Contents

Energy Partners and its Restricted Subsidiaries will no longer be subject to the provisions of the Indenture described above under the caption "— Repurchase at the Option of Holders — Asset Sales" and under the following headings under the caption "— Covenants":

•: "— Restricted Payments,"
•: "— Incurrence of Indebtedness and Issuance of Disqualified Equity,"
•: "— Dividend and Other Payment Restrictions Affecting Subsidiaries,"
•: "— Transactions with Affiliates," and
•: "— Business Activities of MarkWest Finance."

However, MarkWest Energy Partners and its Restricted Subsidiaries shall remain subject to the provisions of the indenture described above under the caption "— Repurchase at the Option of Holders — Change of Control" and described above under the following headings under the caption "— Covenants":

•: "— Liens,"
•: "— Merger, Consolidation or Sale of Assets" (other than the financial test set forth in clause (4) of that covenant),
•: "— Additional Subsidiary Guarantees,"
•: "— Designation of Restricted and Unrestricted Subsidiaries" and
•: "— Reports."

Events of Default and Remedies

Each of the following is an Event of Default:

(1) default for 30 days in the payment when due of interest on the notes;

(2) default in payment when due of the principal of or premium, if any, on the notes;

(3) failure by MarkWest Energy Partners to comply with the provisions described under the caption "— Covenants — Merger, Consolidation or Sale of Assets;"

(4) failure by MarkWest Energy Partners for 90 days after notice to comply with the provisions described under the caption "— Covenants — Reports;"

(5) failure by MarkWest Energy Partners to comply for 30 days after notice with the provisions described under the captions "— Repurchase at the Option of the Holders — Change of Control" or "— Repurchase at the Option of Holders — Asset Sales" or under the covenants described under the caption "— Covenants" above (provided that notice need not be given, and an Event of Default shall occur, 30 days after any breach of the covenants under the captions "— Covenants — Restricted Payments" and "— Covenants — Incurrence of Indebtedness and Issuance of Disqualified Equity") in each case other than a failure to purchase notes which will constitute an Event of Default under clause (2) above and other than a failure to comply with the covenant under the caption "— Covenants — Merger, Consolidation or Sale of Assets" which is covered by clause (3) above and other than a failure to comply with the covenant under the caption "— Covenants — Reports" which is covered by clause (4) above;

(6) failure by MarkWest Energy Partners to comply for 60 days after notice with any of the other agreements in the Indenture;

(7) default under any mortgage, indenture or instrument under which there may be issued or by which there may be secured or evidenced any Indebtedness for money borrowed by an

S-34

Table of Contents

Issuer or any of MarkWest Energy Partners' Restricted Subsidiaries (or the payment of which is guaranteed by MarkWest Energy Partners or any of its Restricted Subsidiaries), whether such Indebtedness or guarantee now exists or is created after the Issue Date, if that default:

(a) is caused by a failure to pay principal of or premium, if any, or interest on such Indebtedness prior to the expiration of the grace period provided in such Indebtedness (a "Payment Default"); or

(b) results in the acceleration of such Indebtedness prior to its express maturity,

and, in each case, the principal amount of any such Indebtedness, together with the principal amount of any other such Indebtedness under which there has been a Payment Default or the maturity of which has been so accelerated, aggregates $75.0 million or more; provided that if any such default is cured or waived or any such acceleration rescinded, or such Indebtedness is repaid, within a period of 30 days from the continuation of such default beyond the applicable grace period or the occurrence of such acceleration, as the case may be, such Event of Default and any consequential acceleration of the notes shall be automatically rescinded, so long as such rescission does not conflict with any judgment or decree;

(8) failure by an Issuer or any of MarkWest Energy Partners' Restricted Subsidiaries to pay final judgments entered by a court of competent jurisdiction aggregating in excess of $75.0 million, which judgments are not paid, discharged or stayed for a period of 60 days;

(9) except as permitted by the Indenture, any Guarantee shall be held in any judicial proceeding to be unenforceable or invalid or shall cease for any reason to be in force and effect or any Subsidiary Guarantor, or any Person acting on behalf of any Subsidiary Guarantor, shall deny or disaffirm its Obligations under its Guarantee; and

(10) certain events of bankruptcy or insolvency with respect to MarkWest Finance, MarkWest Energy Partners or any of its Restricted Subsidiaries that is a Significant Subsidiary or any group of Restricted Subsidiaries that, taken together, would constitute a Significant Subsidiary.

In the case of an Event of Default arising from certain events of bankruptcy or insolvency with respect to the Issuers, all outstanding notes will become due and payable immediately without further action or notice. If any other Event of Default occurs and is continuing, the Trustee or the holders of at least 25% in principal amount of the then outstanding notes may declare all the notes to be due and payable immediately.

Holders of the notes may not enforce the Indenture or the notes except as provided in the Indenture. Subject to certain limitations, holders of a majority in principal amount of the then outstanding notes may direct the Trustee in its exercise of any trust or power. The Trustee may withhold from holders of the notes notice of any continuing Default or Event of Default (except a Default or Event of Default relating to the payment of principal or interest) if it determines that withholding notice is in their interest.

The holders of a majority in principal amount of the notes then outstanding by notice to the Trustee may on behalf of the holders of all of the notes waive any existing Default or Event of Default and its consequences under the Indenture except a continuing Default or Event of Default in the payment of interest on, or the principal of, the notes.

The Issuers and the Subsidiary Guarantors will be required to deliver to the Trustee annually an officers' certificate regarding compliance with the Indenture. Upon any officer of the General Partner or MarkWest Finance becoming aware of any Default or Event of Default, the Issuers must deliver to the Trustee a statement specifying such Default or Event of Default.

S-35

Table of Contents

No Personal Liability of Directors, Officers, Employees and Unitholders

No past, present or future director, officer, partner, employee, incorporator, manager or unitholder or other owner of Equity Interests of the Issuers, the General Partner, or any Subsidiary Guarantor, as such, shall have any liability for any Obligations of the Issuers or the Subsidiary Guarantors under the notes, the Indenture, the Guarantees or for any claim based on, in respect of, or by reason of, such Obligations or their creation. Each holder of notes by accepting a note waives and releases all such liability. The waiver and release are part of the consideration for issuance of the notes. The waiver may not be effective to waive liabilities under the federal securities laws.

Legal Defeasance and Covenant Defeasance

The Issuers may, at their option and at any time, elect to have all of the Issuers' Obligations discharged with respect to the outstanding notes and all Obligations of the Subsidiary Guarantors discharged with respect to their Guarantees ("Legal Defeasance"), except for:

(1) the rights of holders of outstanding notes to receive payments in respect of the principal of, premium, if any, and interest on such notes when such payments are due from the trust referred to below;

(2) the Issuers' obligations with respect to the notes concerning issuing temporary notes, registration of notes, mutilated, destroyed, lost or stolen notes and the maintenance of an office or agency for payment and money for security payments held in trust;

(3) the rights, powers, trusts, duties and immunities of the Trustee, and the Issuers' obligations in connection therewith; and

(4) the Legal Defeasance provisions of the Indenture.

In addition, the Issuers may, at their option and at any time, elect to have the obligations of the Issuers and the Guarantors released with respect to certain covenants that are described in the Indenture ("Covenant Defeasance") and thereafter any omission to comply with those covenants shall not constitute a Default or Event of Default with respect to the notes. In the event Covenant Defeasance occurs, certain events (not including non-payment and bankruptcy or insolvency events) described under "— Events of Default and Remedies" will no longer constitute an Event of Default with respect to the notes. If the Issuers exercise either their Legal Defeasance or Covenant Defeasance option, each Subsidiary Guarantor will be released and relieved of any obligations under its Guarantee and any security for the notes (other than the trust) will be released.

In order to exercise either Legal Defeasance or Covenant Defeasance:

(1) the Issuers must irrevocably deposit with the Trustee, in trust, for the benefit of the holders of the notes, cash in U.S. dollars, U.S. Government Obligations, or a combination thereof, in such amounts as will be sufficient, in the opinion of a nationally recognized firm of independent public accountants, to pay the principal of, premium, if any, and interest on the outstanding notes at the Stated Maturity thereof or on the applicable redemption date, as the case may be, and the Issuers must specify whether the notes are being defeased to Stated Maturity or to a particular redemption date;

(2) in the case of Legal Defeasance, the Issuers shall have delivered to the Trustee an opinion of counsel reasonably acceptable to the Trustee confirming that (a) MarkWest Energy Partners has received from, or there has been published by, the Internal Revenue Service a ruling or (b) since the Issue Date, there has been a change in the applicable federal income tax law, in either case to the effect that, and based thereon such opinion of counsel shall confirm that, the holders of the outstanding notes will not recognize income, gain or loss for federal income tax purposes as a result of such Legal Defeasance and will be subject to federal income tax on the

S-36

Table of Contents

same amounts, in the same manner and at the same times as would have been the case if such Legal Defeasance had not occurred;

(3) in the case of Covenant Defeasance, the Issuers shall have delivered to the Trustee an opinion of counsel reasonably acceptable to the Trustee confirming that the holders of the outstanding notes will not recognize income, gain or loss for federal income tax purposes as a result of such Covenant Defeasance and will be subject to federal income tax on the same amounts, in the same manner and at the same times as would have been the case if such Covenant Defeasance had not occurred;

(4) no Default or Event of Default shall have occurred and be continuing on the date of such deposit (other than a Default or Event of Default resulting from the borrowing of funds to be applied to such deposit (and any similar concurrent deposit relating to other Indebtedness), and the granting of Liens to secure such borrowings);

(5) such Legal Defeasance or Covenant Defeasance will not result in a breach or violation of, or constitute a default under any material agreement or instrument (other than the Indenture or any agreement governing other Indebtedness being defeased, discharged or replaced) to which MarkWest Energy Partners or any of its Subsidiaries is a party or by which MarkWest Energy Partners or any of its Subsidiaries is bound;

(6) the Issuers must deliver to the Trustee an officers' certificate stating that the deposit was not made by the Issuers with the intent of preferring the holders of notes over the other creditors of the Issuers or with the intent of defeating, hindering, delaying or defrauding other creditors of the Issuers; and

(7) the Issuers must deliver to the Trustee an officers' certificate and an opinion of counsel, each stating that all conditions precedent relating to the Legal Defeasance or the Covenant Defeasance have been complied with.

Amendment, Supplement and Waiver

Generally, the Issuers, the Subsidiary Guarantors and the Trustee may amend or supplement the Indenture, the Guarantees and the notes with the consent of the holders of at least a majority in principal amount of the notes then outstanding. However, without the consent of each holder affected, an amendment, supplement or waiver may not (with respect to any notes held by a non-consenting holder):

(1) reduce the principal amount of notes whose holders must consent to an amendment, supplement or waiver;

(2) reduce the principal of or change the fixed maturity of any note or alter or waive the provisions with respect to the redemption or repurchase of the notes (other than provisions relating to minimum required notice of optional redemption or those provisions relating to the covenants described above under the caption "— Repurchase at the Option of Holders");

(3) reduce the rate of or change the time for payment of interest on any note;

(4) waive a Default or Event of Default in the payment of principal of or premium, if any, or interest on the notes (except a rescission of acceleration of the notes by the holders of a majority in principal amount of the notes and a waiver of the payment default that resulted from such acceleration);

(5) make any note payable in money other than that stated in the notes;

(6) make any change in the provisions of the Indenture relating to waivers of past Defaults or the rights of holders of notes to receive payments of principal of or premium, if any, or interest on the notes (other than as permitted in clause (7) below);

S-37

Table of Contents

(7) waive a redemption or repurchase payment with respect to any note (other than a payment required by one of the covenants described above under the caption "— Repurchase at the Option of Holders");

(8) except as otherwise permitted in the Indenture, release any Subsidiary Guarantor from its Obligations under its Guarantee or the Indenture or change any Guarantee in any manner that would adversely affect the rights of holders; or

(9) make any change in the preceding amendment, supplement and waiver provisions (except to increase any percentage set forth therein).

Notwithstanding the preceding, without the consent of any holder of notes, the Issuers, the Subsidiary Guarantors and the Trustee may amend or supplement the Indenture, the Guarantees or the notes:

(1) to cure any ambiguity, defect or inconsistency;

(2) to provide for uncertificated notes in addition to or in place of certificated notes;

(3) to provide for the assumption of an Issuer's or Subsidiary Guarantor's obligations to holders of notes in the case of a merger or consolidation or sale of all or substantially all of such Issuer's or Subsidiary Guarantor's properties or assets, as applicable;

(4) to add or release Subsidiary Guarantors pursuant to the terms of the Indenture;

(5) to make any change that would provide any additional rights or benefits to the holders of notes or surrender any right or power conferred upon the Issuers or the Subsidiary Guarantors by the Indenture that does not materially adversely affect the rights under the Indenture of any holder of the notes, provided that any change to conform the Indenture to this prospectus will not be deemed to materially adversely affect such rights;

(6) to provide for the issuance of additional notes in accordance with the limitations set forth in the Indenture;

(7) to comply with requirements of the SEC in order to effect or maintain the qualification of the Indenture under the Trust Indenture Act;

(8) to evidence or provide for the acceptance of appointment under the Indenture of a successor Trustee;

(9) to add any additional Events of Default;

(10) to secure the notes and/or the Guarantees; or

(11) to provide for the reorganization of MarkWest Energy Partners as any other form of entity, in accordance with the second paragraph of "— Covenants — Merger, Consolidation or Sale of Assets."

Satisfaction and Discharge

The Indenture will be discharged and will cease to be of further effect as to all notes issued thereunder (except as to surviving rights of registration of transfer or exchange of the notes and as otherwise specified in the Indenture), when

(1) either:

(a) all notes that have been authenticated, except lost, stolen or destroyed notes that have been replaced or paid and notes for whose payment money has been deposited in trust and thereafter repaid to the Issuers, have been delivered to the Trustee for cancellation; or

(b) all notes that have not been delivered to the Trustee for cancellation have become due and payable or will become due and payable within one year by reason of the mailing of

S-38

Table of Contents

a notice of redemption or otherwise and the Issuers or any Subsidiary Guarantor has irrevocably deposited or caused to be deposited with the Trustee as trust funds in trust solely for the benefit of the holders, cash in U.S. dollars, U.S. Government Obligations, or a combination of cash in U.S. dollars and U.S. Government Obligations, in amounts as will be sufficient without consideration of any reinvestment of interest, in the opinion of a nationally recognized firm of independent public accountants (in the case of U.S. Government Obligations), to pay and discharge the entire indebtedness on the notes not delivered to the Trustee for cancellation for principal, premium, if any, and accrued interest to the date of fixed maturity or redemption;

(2) in respect of clause (1)(b), no Event of Default has occurred and is continuing on the date of the deposit (other than an Event of Default resulting from the borrowing of funds to be applied to such deposit and any similar deposit relating to other Indebtedness and, in each case, the granting of Liens to secure such borrowings) and the deposit will not result in a breach or violation of, or constitute a default under, any material agreement or instrument to which MarkWest Energy Partners or any of its Subsidiaries is a party or by which MarkWest Energy Partners or any of its Subsidiaries is bound (other than with respect to the borrowing of funds to be applied concurrently to make the deposit required to effect such satisfaction and discharge and any similar concurrent deposit relating to other Indebtedness, and in each case the granting of Liens to secure such borrowings);

(3) the Issuers or any Subsidiary Guarantor has paid or caused to be paid all sums payable by the Issuers under the Indenture; and

(4) the Issuers have delivered irrevocable instructions to the Trustee to apply the deposited money toward the payment of the notes at fixed maturity or the redemption date, as the case may be.

In addition, the Issuers must deliver an officers' certificate and an opinion of counsel to the Trustee stating that all conditions precedent to satisfaction and discharge have been satisfied.

Concerning the Trustee

The Trustee, Wells Fargo Bank, National Association, is also the trustee under the indentures for our outstanding 6.75% Senior Notes due 2020, 6.5% Senior Notes due 2021, 6.25% Senior Notes due 2022, 5.5% Senior Notes due 2023, 4.5% Senior Notes due 2023 and 4.875% Senior Notes due 2024, and it is a lender and administrative agent under the Credit Agreement.

If the Trustee becomes a creditor of an Issuer or any Subsidiary Guarantor, the Indenture will limit its right to obtain payment of claims in certain cases, or to realize on certain property received in respect of any such claim as security or otherwise. The Trustee may engage in other transactions; however, if it acquires any conflicting interest (as defined in the Trust Indenture Act) after a Default has occurred and is continuing it must eliminate such conflict within 90 days, apply to the SEC for permission to continue or resign.

The holders of a majority in principal amount of the then outstanding notes will have the right to direct the time, method and place of conducting any proceeding for exercising any remedy available to the Trustee, subject to certain exceptions. In case an Event of Default occurs and is continuing, the Trustee will be required, in the exercise of its powers, to use the degree of care that a prudent person would use under the circumstances in the conduct of his own affairs. Subject to such provisions, the Trustee will be under no obligation to exercise any of its rights or powers under the Indenture at the request of any holder of notes, unless such holder shall have offered to the Trustee security or indemnity satisfactory to it against any loss, liability or expense.

S-39

Table of Contents

Additional Information

Anyone who receives this prospectus may obtain without charge copies of the base and supplemental indentures that make up the Indenture by writing to MarkWest Energy Partners at 1515 Arapahoe St., Tower 1, Suite 1600, Denver, Colorado 80202, Attention: General Counsel.

Governing Law

The Indenture, the notes and the Guarantees will be governed by, and construed in accordance with, the laws of the State of New York.

Definitions

Set forth below are defined terms used in the Indenture. Reference is made to the Indenture for a full disclosure of all such terms, as well as any other capitalized terms used herein for which no definition is provided.

"Acquired Debt" means, with respect to any specified Person:

(1) Indebtedness of any other Person existing at the time such other Person is merged with or into or became a Subsidiary of such specified Person, whether or not such Indebtedness is incurred in connection with, or in contemplation of, such other Person merging with or into, or becoming a Subsidiary of, such specified Person, but excluding Indebtedness that is extinguished, retired or repaid in connection with such Person merging with or becoming a Subsidiary of such specified Person; and

(2) Indebtedness secured by a Lien encumbering any asset acquired by such specified Person.

"Affiliate" of any specified Person means any other Person directly or indirectly controlling or controlled by or under direct or indirect common control with such specified Person. For purposes of this definition, "control," as used with respect to any Person, shall mean the possession, directly or indirectly, of the power to direct or cause the direction of the management or policies of such Person, whether through the ownership of voting securities, by agreement or otherwise. For purposes of this definition, the terms "controlling," "controlled by" and "under common control with" shall have correlative meanings. Notwithstanding the preceding, the term "Affiliate" shall not include a Restricted Subsidiary of any specified Person.

"Asset Sale" means:

(1) the sale, lease, conveyance or other disposition of any assets, other than dispositions of any products, equipment or services in the ordinary course of business; provided that the sale, lease, conveyance or other disposition of all or substantially all of the assets of MarkWest Energy Partners and its Restricted Subsidiaries taken as a whole will be governed by the provisions of the Indenture described above under the caption "— Repurchase at the Option of Holders — Change of Control," and/or the provisions described above under the caption "— Covenants — Merger, Consolidation or Sale of Assets" and not by the provisions of the Asset Sale covenant; and

(2) the issuance of Equity Interests by any of MarkWest Energy Partners' Restricted Subsidiaries or the sale by MarkWest Energy Partners or any of its Restricted Subsidiaries of Equity Interests in any of its Restricted Subsidiaries.

Notwithstanding the preceding, the following items shall not be deemed to be Asset Sales:

(1) any single transaction or series of related transactions that: (a) involves assets having a fair market value of less than $25.0 million; or (b) results in net proceeds to MarkWest Energy Partners and its Restricted Subsidiaries of less than $25.0 million;

S-40

Table of Contents

(2) a transfer of assets between or among MarkWest Energy Partners and its Restricted Subsidiaries;

(3) an issuance or sale of Equity Interests by a Restricted Subsidiary to MarkWest Energy Partners or to another Restricted Subsidiary of MarkWest Energy Partners;

(4) a Restricted Payment that is permitted by the covenant described above under the caption "— Covenants — Restricted Payments" or a Permitted Investment;

(5) the sale or other disposition of cash or Cash Equivalents, Hedging Obligations or other financial instruments in the ordinary course of business;

(6) any trade or exchange by MarkWest Energy Partners or any of its Restricted Subsidiaries of assets for properties or assets owned or held by another Person, provided that the fair market value of the assets traded or exchanged by MarkWest Energy Partners or such Restricted Subsidiary (together with any cash) is reasonably equivalent to the fair market value of the assets (together with any cash) to be received by MarkWest Energy Partners or such Restricted Subsidiary, and provided further that any cash received must be applied in accordance with the provisions of the Asset Sales covenant;

(7) surrender or waiver of contract rights or the settlement, release or surrender of contract, tort or other claims of any kind;

(8) the creation or perfection of a Lien that is not prohibited by the covenant described above under the caption "— Covenants — Liens;"

(9) dispositions in connection with Permitted Liens; and

(10) the grant in the ordinary course of business of any non-exclusive license of patents, trademarks, registrations therefor and other similar intellectual property.

"Available Cash" has the meaning assigned to such term in the Partnership Agreement, as in effect on the Issue Date.

"Beneficial Owner" has the meaning assigned to such term in Rule 13d-3 and Rule 13d-5 under the Exchange Act, except that in calculating the beneficial ownership of any particular "person" (as that term is used in Section 13(d)(3) of the Exchange Act), such "person" will be deemed to have beneficial ownership of all securities that such "person" has the right to acquire by conversion or exercise of other securities, whether such right is currently exercisable or is exercisable only upon the occurrence of a subsequent condition. The terms "Beneficially Owns" and "Beneficially Owned" have correlative meanings. For purposes of this definition, a Person shall be deemed not to Beneficially Own securities that are the subject of a stock purchase agreement, merger agreement, amalgamation agreement, arrangement agreement or similar agreement until consummation of the transactions or, as applicable, series of related transactions contemplated thereby.

"Board of Directors" means, with respect to MarkWest Energy Partners, the Board of Directors of the General Partner, or any authorized committee of such Board of Directors, and with respect to MarkWest Finance or any other Subsidiary of the Partnership, the Board of Directors or managing members of such Person.

"Board Resolution" means a copy of a resolution certified by the Secretary or an Assistant Secretary of the applicable Person to have been duly adopted by the Board of Directors of such Person and to be in full force and effect on the date of such certification, and delivered to the Trustee.

"Capital Lease Obligation" means, at the time any determination thereof is to be made, the amount of the liability in respect of a capital lease that would at that time be required to be capitalized on a balance sheet in accordance with GAAP. Notwithstanding the foregoing, any lease (whether entered

S-41

Table of Contents

into before or after the Issue Date) that would have been classified as an operating lease pursuant to GAAP as in effect on the Issue Date will be deemed not to represent a Capital Lease Obligation.

"Cash Equivalents" means:

(1) U.S. dollars or, in an amount up to the amount necessary or appropriate to fund local operating expenses, other currencies;

(2) securities issued or directly and fully guaranteed or insured by the U.S. government or any agency or instrumentality thereof (provided that the full faith and credit of the United States is pledged in support thereof) having maturities of not more than one year from the date of acquisition;

(3) certificates of deposit, time deposits and eurodollar time deposits with maturities of one year or less from the date of acquisition, bankers' acceptances with maturities not exceeding 365 days, demand and overnight bank deposits and other similar types of investments routinely offered by commercial banks, in each case, with any domestic commercial bank having a combined capital and surplus in excess of $500.0 million and a Thomson BankWatch Rating of "B" or better or any commercial bank of any other country that is a member of the Organization for Economic Cooperation and Development ("OECD") and has total assets in excess of $500.0 million;

(4) repurchase obligations with a term of not more than seven days for underlying securities of the types described in clauses (2) and (3) above entered into with any financial institution meeting the qualifications specified in clause (3) above;

(5) commercial paper having one of the two highest ratings obtainable from Moody's or Standard & Poor's and in each case maturing within six months after the date of acquisition; and

(6) money market funds at least 95% of the assets of which constitute Cash Equivalents of the kinds described in clauses (1) through (5) of this definition.

"Change of Control" means the occurrence of any of the following:

(1) the direct or indirect lease, sale, transfer, conveyance or other disposition (other than by way of merger or consolidation), in one or a series of related transactions, of all or substantially all of the properties or assets (including Equity Interests of the Restricted Subsidiaries) of MarkWest Energy Partners and its Restricted Subsidiaries taken as a whole, to any "person" (as that term is used in Section 13(d)(3) of the Exchange Act), which disposition is followed by a Rating Decline within 90 days thereafter;

(2) the adoption of a plan relating to the liquidation or dissolution of MarkWest Energy Partners or the removal of the General Partner by the limited partners of MarkWest Energy Partners;

(3) the consummation of any transaction (including, without limitation, any merger or consolidation) the result of which is that any "person" (as that term is used in Section 13(d)(3) of the Exchange Act), excluding the MarkWest Group, becomes the Beneficial Owner, directly or indirectly, of more than 50% of the Voting Stock of the General Partner, measured by voting power rather than number of shares, units or the like, which occurrence is followed by a Rating Decline within 90 days thereafter;

(4) the consummation of any transaction (including, without limitation, any merger or consolidation) the result of which is that any "person" (as that term is used in Section 13(d)(3) of the Exchange Act) becomes the Beneficial Owner, directly or indirectly, of more than 50% of the Voting Stock of MarkWest Energy Partners, measured by voting power rather than number of shares, units or the like, at a time when MarkWest Energy Partners still Beneficially Owns more than 50% of the Voting Stock of the General Partner, measured by voting power rather than number of shares, units or the like, which occurrence is followed by a Rating Decline within 90 days thereafter; or

S-42

Table of Contents

(5) the first day on which a majority of the members of the Board of Directors of the General Partner are not Continuing Directors, which occurrence is followed by a Rating Decline within 90 days thereafter.

Notwithstanding the preceding, a conversion of MarkWest Energy Partners from a limited partnership to a corporation, limited liability company or other form of entity or an exchange of all of the outstanding limited partnership interests for capital stock in a corporation, for member interests in a limited liability company or for Equity Interests in such other form of entity shall not constitute a Change of Control, so long as following such conversion or exchange the MarkWest Group Beneficially Owns, directly or indirectly, in the aggregate more than 50% of the Voting Stock of such entity, or continues to Beneficially Own a sufficient percentage of Voting Stock of such entity to elect a majority of its directors, managers, trustees or other persons serving in a similar capacity for such entity.

"Code" means the Internal Revenue Code of 1986, as amended from time to time, and the rules and regulations thereunder, and any successor thereto.

"Consolidated Cash Flow" means, with respect to any Person for any period, the Consolidated Net Income of such Person for such period plus:

(1) an amount equal to any net loss realized by such Person and its Restricted Subsidiaries in connection with an Asset Sale, to the extent such losses were deducted in computing such Consolidated Net Income; plus

(2) provision for taxes based on income or profits of such Person and its Restricted Subsidiaries for such period, to the extent that such provision for taxes was deducted in computing such Consolidated Net Income; plus

(3) the consolidated interest expense of such Person and its Restricted Subsidiaries for such period, whether paid or accrued (including, without limitation, amortization of debt issuance costs and original issue discount, non-cash interest payments, the interest component of any deferred payment obligations, the interest component of all payments associated with Capital Lease Obligations, commissions, discounts and other fees and charges incurred in respect of letter of credit or bankers' acceptance financings), and net of the effect of all payments, made or received pursuant to interest-rate Hedging Obligations, to the extent that any such expense was deducted in computing such Consolidated Net Income; plus

(4) depreciation, depletion and amortization (including amortization of goodwill and other intangibles but excluding amortization of prepaid cash expenses that were paid in a prior period) and other non-cash expenses (excluding any such non-cash expense to the extent that it represents an accrual of or reserve for cash expenses in any future period or amortization of a prepaid cash expense that was paid in a prior period) of such Person and its Restricted Subsidiaries for such period to the extent that such depreciation, depletion, amortization, impairment and other non-cash expenses were deducted in computing such Consolidated Net Income; plus

(5) unrealized non-cash losses resulting from foreign currency balance sheet adjustments required by GAAP to the extent such losses are deducted in computing such Consolidated Net Income; plus

(6) all extraordinary, unusual or non-recurring items of gain or loss, or revenue or expense; minus

(7) non-cash items increasing such Consolidated Net Income for such period, other than items that were accrued in the ordinary course of business;

in each case, on a consolidated basis and determined in accordance with GAAP.

Notwithstanding the preceding, the provision for taxes based on the income or profits of, and the depreciation, depletion and amortization and other non-cash charges of, a Restricted Subsidiary of

S-43

Table of Contents

MarkWest Energy Partners shall be added to Consolidated Net Income to compute Consolidated Cash Flow of MarkWest Energy Partners only to the extent that a corresponding amount would be permitted at the date of determination to be dividended or distributed to MarkWest Energy Partners by such Restricted Subsidiary without prior approval (that has not been obtained), pursuant to the terms of its charter and all agreements (other than the Indenture, the notes or its Guarantee), instruments, judgments, decrees, orders, statutes, rules and governmental regulations applicable to that Restricted Subsidiary or its stockholders, partners or members.

"Consolidated Net Income" means, with respect to any specified Person for any period, the aggregate of the Net Income of such Person and its Restricted Subsidiaries for such period, on a consolidated basis, determined in accordance with GAAP; provided that:

(1) the Net Income (but not loss) of any Person that is not a Restricted Subsidiary or that is accounted for by the equity method of accounting will be included, but only to the extent of the amount of dividends or distributions paid in cash to the specified Person or a Restricted Subsidiary of the Person;

(2) the Net Income of any Restricted Subsidiary shall be excluded to the extent that the declaration or payment of dividends or similar distributions by that Restricted Subsidiary of that Net Income is not at the date of determination permitted without any prior governmental approval (that has not been obtained) or, directly or indirectly, by operation of the terms of its charter or any agreement (other than the Indenture, the notes or its Guarantee), instrument, judgment, decree, order, statute, rule or governmental regulation applicable to that Restricted Subsidiary or its stockholders, partners or members;

(3) the cumulative effect of a change in accounting principles shall be excluded;

(4) unrealized losses and gains under derivative instruments included in the determination of Consolidated Net Income, including, without limitation, those resulting from the application of Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) 815, shall be excluded; and

(5) any charges relating to any premium or penalty paid, write off of deferred finance costs or other charges in connection with redeeming or retiring any Indebtedness prior to its Stated Maturity shall be excluded.

"Consolidated Net Tangible Assets" means, with respect to any Person at any date of determination, the aggregate amount of total assets included in such Person's most recent quarterly or annual consolidated balance sheet prepared in accordance with GAAP less applicable reserves reflected in such balance sheet, after deducting the following amounts: (1) all current liabilities reflected in such balance sheet, and (2) all goodwill, trademarks, patents, unamortized debt discounts and expenses and other like intangibles reflected in such balance sheet.

"continuing" means, with respect to any Default or Event of Default, that such Default or Event of Default has not been cured or waived.

"Continuing Directors" means, as of any date of determination, any member of the Board of Directors of the General Partner who (1) was a member of such Board of Directors on the Issue Date or (2) was nominated for election or elected to such Board of Directors with the approval of a majority of the Continuing Directors who were members of such Board at the time of such nomination or election.

"Credit Agreement" means that certain Amended and Restated Credit Agreement, dated July 1, 2010, among MarkWest Energy Partners, the banks parties thereto and Wells Fargo Bank, National Association, as administrative agent, consisting of a revolver loan, including any related notes, guarantees, collateral documents, instruments and agreements executed in connection therewith, and in each case as amended, restated, modified, renewed, refunded, replaced or refinanced in whole or in part from time to time.

S-44

Table of Contents

"Credit Facilities" means, with respect to MarkWest Energy Partners, MarkWest Finance or any Restricted Subsidiary, one or more credit facilities, indentures or commercial paper facilities, including the Credit Agreement, providing for revolving credit loans, term loans, capital market financings, private placements, receivables financing (including through the sale of receivables to lenders or to special purpose entities formed to borrow from lenders against such receivables) or letters of credit, in each case, as amended, restated, modified, renewed, refunded, replaced or refinanced in whole or in part from time to time.

"Customary Recourse Exceptions" means, with respect to any Non-Recourse Debt of an Unrestricted Subsidiary, exclusions from the exculpation provisions with respect to such Non-Recourse Debt for fraud, misapplication of cash, waste, willful destruction, bad faith and other circumstances customarily excluded by lenders from exculpation provisions or included in separate indemnification agreements in non-recourse financings.

"Default" means any event that is, or with the passage of time or the giving of notice or both would be, an Event of Default.

"Disqualified Equity" means any Equity Interest that, by its terms (or by the terms of any security into which it is convertible, or for which it is exchangeable, in each case at the option of the holder thereof), or upon the happening of any event, matures or is mandatorily redeemable, pursuant to a sinking fund obligation or otherwise, or redeemable at the option of the holder thereof, in whole or in part, on or prior to the date that is 91 days after the date' on which the notes mature. Notwithstanding the preceding sentence, any Equity Interest that would constitute Disqualified Equity solely because the holders thereof have the right to require MarkWest Energy Partners or any of its Restricted Subsidiaries to repurchase such Equity Interest upon the occurrence of a change of control or an asset sale shall not constitute Disqualified Equity if the terms of such Equity Interest provide that MarkWest Energy Partners or Restricted Subsidiary may not repurchase or redeem any such Equity Interest pursuant to such provisions unless such repurchase or redemption complies with the covenant described above under the caption "— Covenants — Restricted Payments."

"Domestic Subsidiary" means any Restricted Subsidiary of MarkWest Energy Partners that was formed under the laws of the United States or any state of the United States or the District of Columbia.

"Equity Interests" means:

(1) in the case of a corporation, corporate stock;

(2) in the case of an association or business entity, any and all shares, interests, participations, rights or other equivalents (however designated) of corporate stock;

(3) in the case of a partnership or limited liability company, partnership or membership interests (whether general or limited);

(4) any other interest or participation that confers on a Person the right to receive a share of the profits and losses of, or distributions of assets of, the issuing Person; and

(5) all warrants, options or other rights to acquire any of the interests described in clauses (1)-(4) above (but excluding any debt security that is convertible into, or exchangeable for, any of the interests described in clauses (1)-(4) above).

"Existing Indebtedness" means the aggregate principal amount of Indebtedness of MarkWest Energy Partners and its Restricted Subsidiaries in existence on the Issue Date.

The term "fair market value" means the value that would be paid by a willing buyer to an unaffiliated willing seller in a transaction not involving distress or necessity of either party, determined in good faith by the Board of Directors of the General Partner in the case of amounts of $50.0 million or more and otherwise by an officer of the General Partner.

S-45

Table of Contents

"Fixed Charge Coverage Ratio" means, with respect to any specified Person for any four-quarter reference period, the ratio of the Consolidated Cash Flow of such Person for such period to the Fixed Charges of such Person for such period. In the event that the specified Person or any of its Restricted Subsidiaries incurs, assumes, guarantees, repays, redeems, defeases or otherwise retires any Indebtedness (other than revolving credit borrowings not constituting a permanent commitment reduction) or issues, repurchases or redeems Disqualified Equity subsequent to the commencement of the period for which the Fixed Charge Coverage Ratio is being calculated but prior to the date on which the event for which the calculation of the Fixed Charge Coverage Ratio is made (the "Calculation Date"), then the Fixed Charge Coverage Ratio shall be calculated giving pro forma effect to such incurrence, assumption, guarantee, repayment, redemption, defeasance or other retirement of Indebtedness, or such issuance, repurchase or redemption of Disqualified Equity, and the application of the net proceeds thereof as if the same had occurred at the beginning of the applicable four-quarter reference period.

In addition, for purposes of calculating the Fixed Charge Coverage Ratio:

(1) acquisitions that have been made by the specified Person or any of its Restricted Subsidiaries, including through mergers, consolidations or otherwise (including acquisitions of assets used in a Permitted Business), and including in each case any related financing transactions (including repayment of Indebtedness) during the four-quarter reference period or subsequent to such reference period and on or prior to the Calculation Date, will be given pro forma effect as if they had occurred on the first day of the four-quarter reference period, including any Consolidated Cash Flow and any pro forma expense and cost reductions that have occurred or are reasonably expected to occur, in the reasonable judgment of the chief financial or accounting officer of such Person (regardless of whether those cost savings or operating improvements could then be reflected in pro forma financial statements in accordance with Regulation S-X promulgated under the Securities Act or any other regulation or policy of the SEC related thereto);

(2) the Consolidated Cash Flow attributable to discontinued operations, as determined in accordance with GAAP, and operations or businesses disposed of prior to the Calculation Date, shall be excluded;

(3) the Fixed Charges attributable to discontinued operations, as determined in accordance with GAAP, and operations or businesses disposed of prior to the Calculation Date, shall be excluded, but only to the extent that the obligations giving rise to such Fixed Charges will not be obligations of the specified Person or any of its Restricted Subsidiaries following the Calculation Date;

(4) interest on outstanding Indebtedness of the specified Person or any of its Restricted Subsidiaries as of the last day of the four-quarter reference period shall be deemed to have accrued at a fixed rate per annum equal to the rate of interest on such Indebtedness in effect on such last day after giving effect to any Hedging Obligation then in effect; and

(5) if interest on any Indebtedness incurred by the specified Person or any of its Restricted Subsidiaries on such date may optionally be determined at an interest rate based upon a factor of a prime or similar rate, a eurocurrency interbank offered rate or other rates, then the interest rate in effect on the last day of the four-quarter reference period will be deemed to have been in effect during such period.

"Fixed Charges" means, with respect to any Person for any period, the sum, without duplication, of:

(1) the consolidated interest expense of such Person and its Restricted Subsidiaries for such period, whether paid or accrued, including, without limitation, amortization of debt issuance costs and original issue discount, non-cash interest payments, the interest component of any deferred payment obligations, the interest component of all payments associated with Capital Lease Obligations, commissions, discounts, and other fees and charges incurred in respect of letter

S-46

Table of Contents

of credit or bankers' acceptance financings, and net of the effect of all payments made or received pursuant to interest-rate Hedging Obligations; plus

(2) the consolidated interest expense of such Person and its Restricted Subsidiaries that was capitalized during such period; plus

(3) any interest expense on Indebtedness of another Person that is guaranteed by such Person or one of its Restricted Subsidiaries or secured by a Lien on assets of such Person or one of its Restricted Subsidiaries, whether or not such guarantee or Lien is called upon; plus

(4) all dividends, whether paid or accrued and whether or not in cash, on any series of Disqualified Equity of such Person or any of its Restricted Subsidiaries, other than dividends on Equity Interests payable solely in Equity Interests of such Person (other than Disqualified Equity) or to such Person or a Restricted Subsidiary of such Person;

in each case, on a consolidated basis and determined in accordance with GAAP.

"Foreign Subsidiary" means any Restricted Subsidiary of the Company that (a) is not a Domestic Subsidiary and (b) has 50% or more of its assets located outside the United States or any territory thereof.

"GAAP" means generally accepted accounting principles in the United States, which are in effect from time to time.

"General Partner" means MarkWest Energy GP, L.L.C., a Delaware limited liability company, and its successors and permitted assigns as general partner of MarkWest Energy Partners.

The term "guarantee" means a guarantee, other than by endorsement of negotiable instruments for collection in the ordinary course of business, direct or indirect, in any manner, including, without limitation, by way of a pledge of assets, or through letters of credit or reimbursement, "claw-back," "make-well," or "keep-well" agreements in respect thereof, of all or any part of any Indebtedness. When used as a verb, "guarantee" has a correlative meaning.

"Guarantor Subordinated Obligation" means, with respect to a Subsidiary Guarantor, any Indebtedness or other Obligations of such Subsidiary Guarantor (whether outstanding on the Issue Date or thereafter incurred) which are expressly subordinate in right of payment to the Obligations of such Subsidiary Guarantor under its Guarantee pursuant to a written agreement.

"Hedging Obligations" means, with respect to any Person, the obligations of such Person under interest rate and commodity price swap agreements, interest rate and commodity price cap agreements, interest rate and commodity price collar agreements and foreign currency and commodity price exchange agreements, options or futures contracts or other similar agreements or arrangements or Hydrocarbon hedge contracts or Hydrocarbon forward sales contracts, in each case designed to protect such Person against fluctuations in interest rates, foreign exchange rates, or commodities prices.

"Hydrocarbons" means crude oil, natural gas, casinghead gas, drip gasoline, natural gasoline, condensate, distillate, liquid hydrocarbons, gaseous hydrocarbons and all constituents, elements or compounds thereof and products refined or processed therefrom.

"Indebtedness" means, with respect to any specified Person, any indebtedness of such Person, whether or not contingent:

(1) in respect of borrowed money;

(2) evidenced by bonds, notes, debentures or similar instruments or letters of credit (or reimbursement agreements in respect thereof);

(3) in respect of bankers' acceptances;

(4) representing Capital Lease Obligations;

S-47

Table of Contents

(5) representing the balance deferred and unpaid of the purchase price of any property, except any such balance that constitutes an accrued expense or trade payable incurred in the ordinary course of business;

(6) representing Disqualified Equity; or

(7) representing any Hedging Obligations;

if and to the extent any of the preceding items (other than letters of credit, Disqualified Equity and Hedging Obligations) would appear as a liability upon a balance sheet of the specified Person prepared in accordance with GAAP. In addition, the term "Indebtedness" includes all Indebtedness of others secured by a Lien on any asset of the specified Person (whether or not such Indebtedness is assumed by the specified Person) and, to the extent not otherwise included, the guarantee by such Person of any Indebtedness of any other Person, provided that a guarantee otherwise permitted by the Indenture to be incurred by MarkWest Energy Partners or any of its Restricted Subsidiaries of Indebtedness incurred by MarkWest Energy Partners or a Restricted Subsidiary in compliance with the terms of the Indenture shall not constitute a separate incurrence of Indebtedness. The term "Indebtedness" excludes, however, any repayment or reimbursement obligation of such Person or any of its Restricted Subsidiaries with respect to Customary Recourse Exceptions, unless and until an event or circumstance occurs that triggers the Person's or such Restricted Subsidiary's direct repayment or reimbursement obligation (as opposed to contingent or performance obligations) to the lender of other Person to whom such obligation is actually owed, in which case the amount of such direct payment or reimbursement obligation shall constitute Indebtedness.

The amount of any Indebtedness outstanding as of any date shall be:

(1) the accreted value thereof, in the case of any Indebtedness issued with original issue discount;

(2) in the case of any Hedging Obligation, the termination value of the agreement or arrangement giving rise to such Hedging Obligation that would be payable by such Person at such date;

(3) in the case of any letter of credit, the maximum potential liability thereunder; and

(4) the principal amount thereof, together with any interest thereon that is more than 30 days past due, in the case of any other Indebtedness.

For purposes of clause (6) of the first paragraph of this definition, Disqualified Equity shall be valued at the maximum fixed redemption, repayment or repurchase price, which shall be calculated in accordance with the terms of such Disqualified Equity as if such Disqualified Equity were repurchased on any date on which Indebtedness shall be required to be determined pursuant to the Indenture; provided, however, that if such Disqualified Equity is not then permitted by its terms to be redeemed, repaid or repurchased, the redemption, repayment or repurchase price shall be the book value of such Disqualified Equity. The amount of Indebtedness of any Person at any date shall be the outstanding balance at such date of all unconditional Obligations as described above and the maximum liability of any guarantees at such date; provided that for purposes of calculating the amount of any non-interest bearing or other discount security, such Indebtedness shall be deemed to be the principal amount thereof that would be shown on the balance sheet of the issuer thereof dated such date prepared in accordance with GAAP, but that such security shall be deemed to have been incurred only on the date of the original issuance thereof.

"Investment Grade Rating" means a rating equal to or higher than Baa3 (or the equivalent) by Moody's or BBB– (or the equivalent) by Standard & Poor's.

"Investments" means, with respect to any Person, all investments by such Person in other Persons (including Affiliates) in the forms of direct or indirect loans (including guarantees of Indebtedness or

S-48

Table of Contents

other Obligations), advances (other than advances to customers in the ordinary course of business that are recorded as accounts receivable on the balance sheet of the lender and commission, moving, travel and similar advances to officers and employees made in the ordinary course of business) or capital contributions, purchases or other acquisitions for consideration of Indebtedness, Equity Interests or other securities, together with all items that are or would be classified as investments on a balance sheet prepared in accordance with GAAP. For purposes of the definition of "Unrestricted Subsidiary," the definition of "Restricted Payment" and the covenant described under the caption "— Covenants — Restricted Payments", (1) the term "Investment" shall include the portion (proportionate to MarkWest Energy Partners' Equity Interest in such Subsidiary) of the fair market value of the net assets of any Subsidiary of MarkWest Energy Partners or any of its Restricted Subsidiaries at the time that such Subsidiary is designated an Unrestricted Subsidiary; provided, however, that upon a redesignation of such Subsidiary as a Restricted Subsidiary, MarkWest Energy Partners or such Restricted Subsidiary shall be deemed to continue to have a permanent "Investment" in such Subsidiary at the time immediately before the effectiveness of such redesignation less the portion (proportionate to MarkWest Energy Partners' or such Restricted Subsidiary's Equity Interest in such Subsidiary) of the fair market value of the net assets of such Subsidiary at the time of such redesignation, and (2) any property transferred to or from an Unrestricted Subsidiary shall be valued at its fair market value at the time of such transfer, in each case as determined in good faith by the Board of Directors of the General Partner. If MarkWest Energy Partners or any Restricted Subsidiary of MarkWest Energy Partners sells or otherwise disposes of any Equity Interests of any direct or indirect Restricted Subsidiary of MarkWest Energy Partners such that, after giving effect to any such sale or disposition, such Person is no longer a Restricted Subsidiary of MarkWest Energy Partners, MarkWest Energy Partners shall be deemed to have made an Investment on the date of any such sale or disposition equal to the fair market value of the Equity Interests of such Restricted Subsidiary not sold or disposed of.

"Issue Date" means the date of the first issuance of notes under the Indenture.

"Joint Venture" means any Person that is not a direct or indirect Subsidiary of MarkWest Energy Partners in which MarkWest Energy Partners or any of its Restricted Subsidiaries makes any Investment.

"Lien" means, with respect to any asset, any mortgage, lien (statutory or otherwise), pledge, charge, security interest, hypothecation, assignment for security, claim, preference, priority or encumbrance of any kind in respect of such asset, whether or not filed, recorded or otherwise perfected under applicable law, including any conditional sale or other title retention agreement or any lease in the nature thereof, any option or other agreement to grant a security interest in and any filing of or agreement to give any financing statement under the Uniform Commercial Code (or equivalent statute) of any jurisdiction other than a precautionary financing statement respecting a lease not intended as a security agreement.

"MarkWest Group" means, collectively, MarkWest Energy Partners and each Person which is a direct or indirect Subsidiary of MarkWest Energy Partners.

"Moody's" means Moody's Investors Service, Inc. or any successor to the rating agency business thereof.

"Net Income" means, with respect to any Person, the consolidated net income (loss) of such Person and its Restricted Subsidiaries, determined in accordance with GAAP and before any reduction in respect of preferred stock dividends, excluding, however:

(1) the aggregate gain (but not loss in excess of such aggregate gain), together with any related provision for taxes on such gain, realized in connection with:

(a) any Asset Sale; or

(b) the disposition of any securities by such Person or any of its Restricted Subsidiaries or the extinguishment of any Indebtedness of such Person or any of its Restricted Subsidiaries; and

S-49

Table of Contents

(2) any extraordinary gain (but not loss), together with any related provision for taxes on such extraordinary gain (but not loss).

"Net Proceeds" means, with respect to any Asset Sale or sale of Equity Interests, the aggregate proceeds received by MarkWest Energy Partners or any of its Restricted Subsidiaries in cash or Cash Equivalents in respect of any Asset Sale or sale of Equity Interests (including, without limitation, any cash received upon the sale or other disposition of any non-cash consideration received in any such sale but excluding any non-cash consideration deemed to be cash for purposes of the "Asset Sales" provisions of the Indenture), net of, without duplication, (1) the direct costs relating to such Asset Sale or sale of Equity Interests, including, without limitation, brokerage commissions and legal, accounting and investment banking fees, sales commissions, recording fees, title transfer fees, and any relocation expenses incurred as a result thereof, (2) taxes paid or payable as a result thereof, in each case after taking into account any available tax credits or deductions and any tax sharing arrangements and amounts required to be applied to the repayment of Indebtedness secured by a Lien on the asset or Equity Interests that were the subject of such Asset Sale or sale of Equity Interests, (3) all distributions and payments required to be made to minority interest holders in Restricted Subsidiaries as a result of such Asset Sale and (4) any amounts to be set aside in any reserve established in accordance with GAAP or any amount placed in escrow, in either case for adjustment in respect of the sale price of such asset or Equity Interests or for liabilities associated with such Asset Sale or sale of Equity Interests and retained by MarkWest Energy Partners or any of its Restricted Subsidiaries until such time as such reserve is reversed or such escrow arrangement is terminated, in which case Net Proceeds shall include only the amount of the reserve so reversed or the amount returned to MarkWest Energy Partners or its Restricted Subsidiaries from such escrow arrangement, as the case may be.

"Non-Recourse Debt" means Indebtedness as to which:

(1) neither MarkWest Energy Partners nor any of its Restricted Subsidiaries (a) provides credit support of any kind (including any undertaking, agreement or instrument that would constitute Indebtedness), (b) is directly or indirectly liable as a guarantor or otherwise, except for Customary Recourse Exceptions, or (c) constitutes the lender of such Indebtedness; and

(2) as to which the lenders will not have any recourse to the stock or assets of MarkWest Energy Partners or any of its Restricted Subsidiaries, except as contemplated by clause (14) of the definition of "Permitted Liens," and except for Customary Recourse Exceptions.

"Obligations" means any principal, interest, penalties, fees, indemnifications, reimbursement obligations, damages and other liabilities payable under the documentation governing any Indebtedness.

"Partnership Agreement" means the Third Amended and Restated Agreement of Limited Partnership of MarkWest Energy Partners, L.P., dated as of February 21, 2008, as such may be amended, modified or supplemented from time to time.

"Permitted Business" means either (1) gathering, transporting, treating, processing, marketing or otherwise handling Hydrocarbons, or activities or services reasonably related or ancillary thereto including entering into Hedging Obligations to support these businesses, or (2) any other business that generates gross income that constitutes "qualifying income" under Section 7704(d) of the Code.

"Permitted Business Investments" means Investments by MarkWest Energy Partners or any of its Restricted Subsidiaries in any Unrestricted Subsidiary of MarkWest Energy Partners or in any Joint Venture, provided that:

(1) either (a) at the time of such Investment and immediately thereafter, MarkWest Energy Partners could incur $1.00 of additional indebtedness under the first paragraph in the limitation of Indebtedness set forth under the caption "— Covenants — Incurrence of Indebtedness and Issuance of Disqualified Equity" above or (b) such Investment does not exceed the aggregate amount of

S-50

Table of Contents

Incremental Funds (as defined in the covenant described under the caption "— Covenants — Restricted Payments") not previously expended at the time of making such Investment;

(2) if such Unrestricted Subsidiary or Joint Venture has outstanding Indebtedness at the time of such Investment, either (a) all such Indebtedness is Non-Recourse Debt or is owed to MarkWest Energy Partners or one of its Restricted Subsidiaries or (b) it is Indebtedness of such Unrestricted Subsidiary or Joint Venture that could, at the time such Investment is made and, if later, at the time any such Indebtedness is incurred, be incurred by MarkWest Energy Partners and its Restricted Subsidiaries in accordance with the limitation on Indebtedness set forth in the first paragraph under the caption "— Covenants — Incurrence of Indebtedness and Issuance of Disqualified Equity;" and

(3) such Unrestricted Subsidiary's or Joint Venture's activities are not outside the scope of the Permitted Business.

"Permitted Investments" means:

(1) any Permitted Business Investment;

(2) any Investment in, or that results in the creation of, any Restricted Subsidiary of MarkWest Energy Partners;

(3) any Investment in MarkWest Energy Partners or in a Restricted Subsidiary of MarkWest Energy Partners (excluding redemptions, purchases, acquisitions or other retirements of Equity Interests in MarkWest Energy Partners but including purchases of its notes);

(4) any Investment in cash or Cash Equivalents;

(5) any Investment by MarkWest Energy Partners or any Restricted Subsidiary of MarkWest Energy Partners in a Person if as a result of such Investment:

(a) such Person becomes a Restricted Subsidiary of MarkWest Energy Partners; or

(b) such Person is merged, consolidated or amalgamated with or into, or transfers or conveys substantially all of its assets to, or is liquidated into, MarkWest Energy Partners or a Restricted Subsidiary of MarkWest Energy Partners;

(6) any Investment made as a result of the receipt of consideration consisting of other than cash or Cash Equivalents from an Asset Sale that was made pursuant to and in compliance with the covenant described above under the caption "— Repurchase at the Option of Holders — Asset Sales," including pursuant to clause (6) of the items deemed not to be Asset Sales in the definition of that term;

(7) any Investment in a Person solely in exchange for the issuance of Equity Interests (other than Disqualified Equity) of MarkWest Energy Partners;

(8) Investments in stock, obligations or securities received in settlement of debts owing to MarkWest Energy Partners or any of its Restricted Subsidiaries as a result of bankruptcy or insolvency proceedings or upon the foreclosure, perfection or enforcement of any Lien in favor of MarkWest Energy Partners or any such Restricted Subsidiary, in each case as to debt owing to MarkWest Energy Partners or any such Restricted Subsidiary that arose in the ordinary course of business of MarkWest Energy Partners or any such Restricted Subsidiary;

(9) any Investment in Hedging Obligations permitted to be incurred by the covenant described above under the caption "— Covenants — Incurrence of Indebtedness and Issuance of Disqualified Equity"; and

(10) other Investments in any Person engaged in a Permitted Business (other than an Investment in an Unrestricted Subsidiary) having an aggregate fair market value (measured on the

S-51

Table of Contents

date each such Investment was made and without giving effect to subsequent changes in value), when taken together with all other Investments made pursuant to this clause (10) since the Issue Date and existing at the time of the Investment, which is the subject of the determination, was made, not to exceed the greater of (a) $250.0 million and (b) 5.0% of Consolidated Net Tangible Assets; provided, however, that if any Investment pursuant to this clause (10) is made in any Person that is not a Restricted Subsidiary of MarkWest Energy Partners at the date of the making of such Investment and such Person becomes a Restricted Subsidiary of MarkWest Energy Partners after such date, such Investment shall thereafter be deemed to have been made pursuant to clause (2) above and shall cease to have been made pursuant to this clause (10) for so long as such Person continues to be a Restricted Subsidiary of MarkWest Energy Partners.

"Permitted Liens" means:

(1) Liens securing Indebtedness under any of the Credit Facilities;

(2) Liens in favor of MarkWest Energy Partners or any of its Restricted Subsidiaries;

(3) any interest or title of a lessor in the property subject to a Capital Lease Obligation;

(4) Liens on property of a Person existing at the time such Person is merged with or into or consolidated with MarkWest Energy Partners or any Restricted Subsidiary of MarkWest Energy Partners, provided that such Liens were in existence prior to, and were not obtained in contemplation of, such merger or consolidation and do not extend to any assets other than those of the Person merged into or consolidated with MarkWest Energy Partners or such Restricted Subsidiary;

(5) Liens on property existing at the time of acquisition thereof by MarkWest Energy Partners or any Restricted Subsidiary of MarkWest Energy Partners, provided that such Liens were in existence prior to, and were not obtained in contemplation of, such acquisition and relate solely to such property, accessions thereto and the proceeds thereof;

(6) Liens to secure the performance of tenders, bids, leases, statutory obligations, surety or appeal bonds, government contracts, performance bonds or other obligations of a like nature incurred in the ordinary course of business;

(7) Liens on any property or asset acquired, constructed or improved by MarkWest Energy Partners or any Restricted Subsidiary, which (a) are in favor of the seller of such property or asset, in favor of the Person constructing or improving such property or asset, or in favor of the Person that provided the funding for the acquisition, construction or improvement of such property or asset, (b) are created within 360 days after the date of acquisition, construction or improvement, (c) secure the purchase price or construction or improvement cost, as the case may be, of such property or asset in an amount not to exceed the lesser of (i) the cost to MarkWest Energy Partners and its Restricted Subsidiaries of such acquisition, construction or improvement of such asset or property and (ii) 100% of the fair market value of such acquisition, construction or improvement of such property or asset, and (d) are limited to the asset or property so acquired, constructed or improved (including proceeds thereof, accessions thereto and upgrades thereof);

(8) Liens to secure performance of Hedging Obligations of MarkWest Energy Partners or a Restricted Subsidiary;

(9) Liens existing on the Issue Date;

(10) Liens arising under operating agreements, joint venture agreements, partnership agreements, construction agreements, oil and gas leases, farmout agreements, division orders, agreements for the purchase, gathering, processing, sale, transportation or exchange of Hydrocarbons, unitization and pooling declarations and agreements, area of mutual interest agreements and other agreements arising in the ordinary course of MarkWest Energy Partners' or any Restricted Subsidiary's business that are customary in the Permitted Business;

S-52

Table of Contents

(11) Liens securing the Obligations of the Issuers under the notes and the Indenture and of the Subsidiary Guarantors under the Guarantees;

(12) Liens upon specific items of inventory, receivables or other goods or proceeds of MarkWest Energy Partners or any of its Restricted Subsidiaries securing such Person's Obligations in respect of bankers' acceptances or receivables securitizations issued or created for the account of such Person to facilitate the purchase, shipment or storage of such inventory, receivables or other goods or proceeds and permitted by the covenant described under the caption "— Covenants — Incurrence of Indebtedness and Issuance of Disqualified Equity;"

(13) Liens securing any Indebtedness equally and ratably with all Obligations due under the notes or any Guarantee pursuant to a contractual covenant that limits Liens in a manner substantially similar to the covenant described above under the caption "— Covenants — Liens;"

(14) Liens on and pledges of the Equity Interests of any Unrestricted Subsidiary or any Joint Venture owned by MarkWest Energy Partners or any Restricted Subsidiary of MarkWest Energy Partners to the extent securing Non-Recourse Debt or other Indebtedness of such Unrestricted Subsidiary or Joint Venture;

(15) Liens incurred in the ordinary course of business of MarkWest Energy Partners or any Restricted Subsidiary of MarkWest Energy Partners, provided that, after giving effect to any such incurrence, the aggregate principal amount of all indebtedness then outstanding and secured by any Liens incurred pursuant to this clause (15) does not exceed the greater of $250.0 million or 5.0% of the Consolidated Net Tangible Assets of MarkWest Energy Partners; and

(16) any Lien renewing, extending, refinancing or refunding a Lien permitted by clauses (1) through (15) above; provided, however, that (a) the principal amount of Indebtedness secured by such Lien does not exceed the principal amount of such Indebtedness outstanding immediately prior to the renewal, extension, refinancing or refunding of such Lien plus all accrued interest on the Indebtedness secured thereby and the amount of all fees, expenses and premiums incurred in connection therewith, and (b) no assets encumbered by any such Lien other than the assets permitted to be encumbered immediately prior to such renewal, extension, refinancing or refunding are encumbered thereby.

"Permitted Refinancing Indebtedness" means any Indebtedness of MarkWest Energy Partners or any of its Restricted Subsidiaries issued in exchange for, or the net proceeds of which are used to extend, refinance, renew, replace, defease or refund other Indebtedness of MarkWest Energy Partners or any of its Restricted Subsidiaries (other than intercompany Indebtedness); provided that:

(1) the principal amount of such Permitted Refinancing Indebtedness does not exceed the principal amount of, plus accrued interest on, the Indebtedness so extended, refinanced, renewed, replaced, defeased or refunded (plus the amount of necessary fees and expenses incurred in connection therewith and any premiums paid on the Indebtedness so extended, refinanced, renewed, replaced, defeased or refunded);

(2) such Permitted Refinancing Indebtedness has a final maturity date no earlier than the final maturity date of and has a Weighted Average Life to Maturity equal to or greater than the Weighted Average Life to Maturity of, the Indebtedness being extended, refinanced, renewed, replaced, defeased or refunded;

(3) if the Indebtedness being extended, refinanced, renewed, replaced, defeased or refunded is subordinated in right of payment to the notes or the Guarantees, such Permitted Refinancing Indebtedness is subordinated in right of payment to the notes or the Guarantees, as the case may be, on terms at least as favorable to the holders of notes as those contained in the documentation governing the Indebtedness being extended, refinanced, renewed, replaced, defeased or refunded; and

S-53

Table of Contents

(4) such Indebtedness is not incurred (other than by way of a guarantee) by a Restricted Subsidiary (other than MarkWest Finance) if MarkWest Energy Partners is the issuer or other primary obligor on the Indebtedness being extended, refinanced, renewed, replaced, defeased or refunded.

"Person" means any individual, corporation, partnership, joint venture, association, joint-stock company, trust, unincorporated organization, limited liability company or government or any agency or political subdivision thereof or any other entity.

"Rating Agency" means each of Standard & Poor's and Moody's, or if Standard & Poor's or Moody's or both shall not make a rating on the notes publicly available, a nationally recognized statistical rating agency or agencies, as the case may be, selected by the Issuers (as certified by a resolution of the Board of Directors of the General Partner) which shall be substituted for Standard & Poor's or Moody's, or both, as the case may be.

"Rating Category" means:

(1) with respect to S&P, any of the following categories: AAA, AA, A, BBB, BB, B, CCC, CC, C and D (or equivalent successor categories); and

(2) with respect to Moody's, any of the following categories: Aaa, Aa, A, Baa, Ba, B, Caa, Ca, C and D (or equivalent successor categories).

"Rating Decline" means a decrease in the rating of the notes by either Moody's or S&P by one or more gradations (including gradations within Rating Categories as well as between Rating Categories). In determining whether the rating of the notes has decreased by one or more gradations, gradations within Rating Categories, namely + or – for S&P, and 1, 2, and 3 for Moody's, will be taken into account; for example, in the case of S&P, a rating decline either from BB+ to BB or BB– to B+ will constitute a decrease of one gradation.

"Reporting Default" means a Default described in clause (4) under "— Events of Default and Remedies."

"Restricted Investment" means an Investment other than a Permitted Investment.

"Restricted Subsidiary" of a Person means any Subsidiary of the referenced Person that is not an Unrestricted Subsidiary. Notwithstanding anything in the Indenture to the contrary, each of MarkWest Finance and the Operating Company shall be a Restricted Subsidiary of MarkWest Energy Partners.

"Significant Subsidiary" means any Subsidiary that would be a "significant subsidiary" as defined in Article 1, Rule 1-02 of Regulation S-X, promulgated pursuant to the Securities Act and the Exchange Act, as such Regulation is in effect on the Issue Date.

"Standard & Poor's" or "S&P" means Standard & Poor's Ratings Services, a division of The McGraw-Hill Companies, Inc., or any successor to the rating agency business thereof.

"Stated Maturity" means, with respect to any installment of interest or principal on any series of Indebtedness, the date on which such payment of interest or principal was scheduled to be paid in the original documentation governing such Indebtedness, and shall not include any contingent Obligations to repay, redeem or repurchase any such interest or principal prior to the date originally scheduled for the payment thereof.

"Subordinated Obligation" means any Indebtedness of MarkWest Energy Partners or MarkWest Finance (whether outstanding on the Issue Date or thereafter incurred) that is subordinate or junior in right of payment to the notes pursuant to a written agreement.

S-54

Table of Contents

"Subsidiary" means, with respect to any Person:

(1) any corporation, association or other business entity (other than an entity referred to in clause (2) below) of which more than 50% of the total Voting Stock is at the time owned or controlled, directly or indirectly, by such Person or one or more of the other Subsidiaries of that Person (or a combination thereof); and

(2) any partnership (whether general or limited), limited liability company or joint venture (a) the sole general partner or member of which is such Person or a Subsidiary of such Person, or (b) if there is more than a single general partner or member, either (i) the only general partners, members, managing general partners or managing members of which are such Person or one or more Subsidiaries of such Person (or any combination thereof) or (ii) such Person owns or controls, directly or indirectly, a majority of the outstanding general partner interests, member interests or other Voting Stock of such partnership, limited liability company or joint venture, respectively, plus in the case of both subclauses (i) and (ii) of this clause (b) it consolidates the financial results of such partnership, limited liability company or joint venture with its own financial results in accordance with GAAP.

"Subsidiary Guarantors" means:

(1) each Restricted Subsidiary of MarkWest Energy Partners that provides a Guarantee on the Issue Date;

(2) any other Subsidiary of MarkWest Energy Partners that becomes a Subsidiary Guarantor in accordance with the provisions of the Indenture; and

(3) their respective successors and assigns;

in each case until such Subsidiary Guarantor ceases to be such in accordance with the Indenture. Notwithstanding anything in the Indenture to the contrary, MarkWest Finance shall not be a Subsidiary Guarantor.

"2012 Issue Date" means August 10, 2012, the initial date of issuance of the Issuers' 5.5% Senior Notes due 2023.

"U.S. Government Obligations" means securities that are (1) direct Obligations of the United States for the payment of which its full faith and credit is pledged; (2) Obligations of a Person controlled or supervised by and acting as an agency or instrumentality of the United States of America the payment of which is unconditionally guaranteed as a full faith and credit obligation by the United States, which, in either case under clauses (1) or (2) above, are not callable or redeemable at the option of the issuers thereof; or (3) depository receipts issued by a bank or trust company as custodian with respect to any such U.S. Government Obligations or a specific payment of interest on or principal of any such U.S. Government Obligation held by such custodian for the account of the holder of a depository receipt, provided that (except as required by law) such custodian is not authorized to make any deduction from the amount payable to the holder of such depository receipt from any amount received by the custodian in respect of the U.S. Government Obligation evidenced by such depository receipt.

"Unrestricted Subsidiary" means Liberty, a Delaware limited liability company, MarkWest Utica EMG, L.L.C., a Delaware limited liability company, MarkWest Panola Pipeline L.L.C., a Texas limited liability company, West Relay Gathering Company, L.L.C., a Delaware limited liability company and any Subsidiary of MarkWest Energy Partners (other than MarkWest Finance or the Operating Company) that is designated by the Board of Directors of the General Partner as an Unrestricted Subsidiary pursuant to a Board Resolution, but only to the extent that such Subsidiary: (1) has no Indebtedness (other than Non-Recourse Debt) owing to any Person other than MarkWest Energy Partners or any of its Restricted Subsidiaries, except to the extent permitted by subclause (2)(b) of the definition of "Permitted Business

S-55

Table of Contents

Investments"; (2) except as permitted by the covenant described above under the caption "— Covenants — Transactions with Affiliates," is not a party to any agreement, contract, arrangement or understanding with MarkWest Energy Partners or any Restricted Subsidiary of MarkWest Energy Partners unless the terms of any such agreement, contract, arrangement or understanding are no less favorable to MarkWest Energy Partners or such Restricted Subsidiary than those that might be obtained at the time from Persons who are not Affiliates of MarkWest Energy Partners; (3) is a Person with respect to which neither MarkWest Energy Partners nor any of its Restricted Subsidiaries has any direct or indirect obligation (a) to subscribe for additional Equity Interests or (b) to maintain or preserve such Person's financial condition or to cause such Person to achieve any specified levels of operating results; and (4) has not guaranteed or otherwise directly or indirectly provided credit support for any Indebtedness of MarkWest Energy Partners or any of its Restricted Subsidiaries, except to the extent such guarantee or credit support would be released upon such designation. All Subsidiaries of an Unrestricted Subsidiary shall be also Unrestricted Subsidiaries. Notwithstanding anything in the Indenture to the contrary, neither MarkWest Finance nor the Operating Company shall be designated as an Unrestricted Subsidiary.

Any designation of a Subsidiary of MarkWest Energy Partners as an Unrestricted Subsidiary shall be evidenced to the Trustee by filing with the Trustee a Board Resolution giving effect to such designation and an officers' certificate certifying that such designation complied with the preceding conditions and was permitted by the covenant described above under the caption "— Covenants — Restricted Payments." If, at any time, any Unrestricted Subsidiary would fail to meet the preceding requirements as an Unrestricted Subsidiary, it shall thereafter cease to be an Unrestricted Subsidiary for purposes of the Indenture and any Indebtedness of such Subsidiary shall be deemed to be incurred by a Restricted Subsidiary of MarkWest Energy Partners as of such date and, if such Indebtedness is not permitted to be incurred as of such date under the covenant described under the caption "— Covenants — Incurrence of Indebtedness and Issuance of Disqualified Equity," MarkWest Energy Partners shall be in default of such covenant.

"Voting Stock" of any Person as of any date means the Equity Interests of such Person pursuant to which the holders thereof have the general voting power under ordinary circumstances to elect at least a majority of the board of directors, managers, general partners or trustees of such Person (regardless of whether, at the time, Equity Interests of any other class or classes shall have, or might have, voting power by reason of the occurrence of any contingency) or, with respect to a partnership (whether general or limited), any general partner interest in such partnership.

"Weighted Average Life to Maturity" means, when applied to any Indebtedness at any date, the number of years obtained by dividing:

(1) the sum of the products obtained by multiplying (a) the amount of each then remaining installment, sinking fund, serial maturity or other required payments of principal, including payment at final maturity, in respect thereof, by (b) the number of years (calculated to the nearest one-twelfth) that will elapse between such date and the making of such payment; by

(2) the then outstanding principal amount of such Indebtedness.

S-56

Table of Contents

BOOK-ENTRY, DELIVERY AND FORM

The notes initially will be represented by one or more notes in registered, global form without interest coupons (collectively, the "Global notes"). Upon issuance, the Global notes will be:

•: deposited with the Trustee as custodian for The Depository Trust Company ("DTC"), in New York, New York, and
•: registered in the name of DTC or its nominee, in each case for credit to an account of a Direct or Indirect Participant as described below.

Transfer of beneficial interests in any Global notes will be subject to the applicable rules and procedures of DTC and its direct or indirect participants (including, if applicable, those of the Euroclear system ("Euroclear") and the Clearstream system ("Clearstream"), which may change from time to time. The Global notes may be transferred, in whole and not in part, only to another nominee of DTC or to a successor of DTC or its nominee in limited circumstances. Beneficial interests in the Global notes may be exchanged for notes in certificated form in limited circumstances. See the caption "— Transfers of Interest in Global Notes for Certificated Notes."

Depositary Procedures

DTC has advised us that DTC is a limited-purpose trust company created to hold securities for its participating organizations (collectively, the "Direct Participants") and to facilitate the clearance and settlement of transactions in those securities between Direct Participants through electronic book-entry changes in accounts of the Direct Participants. The Direct Participants include securities brokers and dealers, banks, trust companies, clearing corporations and other organizations, including Euroclear and Clearstream. Access to DTC's system is also available to other entities that clear through or maintain a direct or indirect, custodial relationship with a Direct Participant (collectively, the "Indirect Participants").

DTC has advised us that, pursuant to DTC's procedures, (i) upon deposit of the Global notes, DTC will credit the accounts of the Direct Participants designated by the underwriters with portions of the principal amount of the Global notes that have been purchased by them, and (ii) DTC will maintain records of the ownership interests of such Direct Participants in the Global notes and the transfer of ownership interests by and between Direct Participants. DTC will not maintain records of the ownership interests of, or the transfer of ownership interests by and between, Indirect Participants or other owners of beneficial interests in the Global notes. Direct Participants and Indirect Participants must maintain their own records of the ownership interests of, and the transfer of ownership interests by and between, Indirect Participants and other owners of beneficial interests in the Global notes.

Investors in the Global notes may hold their interests therein directly through DTC if they are Direct Participants in DTC or indirectly through organizations that are Direct Participants in DTC, including the depositories for Euroclear and Clearstream. Euroclear Bank N.V./S.A. will act initially as depository for Euroclear, and Clearstream Banking, S.A. will act initially as depository for Clearstream (each a "Nominee" of Euroclear and Clearstream, respectively). Therefore, they will each be recorded on DTC's records as the holders of all ownership interests held by them on behalf of Euroclear and Clearstream, respectively. Euroclear and Clearstream must maintain on their own records the ownership interests, and transfers of ownership interests by and between, their own customers' securities accounts. DTC will not maintain such records. All ownership interests in any Global notes, including those of customers' securities accounts held through Euroclear or Clearstream, may be subject to the procedures and requirements of DTC.

The laws of some jurisdictions may require that certain persons take physical delivery in definitive, certificated form, of securities that they own. This may limit or curtail the ability to transfer a beneficial interest in a Global note to such persons. Because DTC can act only on behalf of Direct Participants,

S-57

Table of Contents

which in turn act on behalf of Indirect Participants and others, the ability of a person having a beneficial interest in a Global note to pledge such interest to persons or entities that are not Direct Participants in DTC, or to otherwise take actions in respect of such interest, may be affected by the lack of physical certificates evidencing such interest.

Except as described under the caption "— Transfers of Interest in Global Notes for Certificated Notes," owners of beneficial interests in the Global notes will not have notes registered in their names, will not receive physical delivery of notes in certificated form and will not be considered the registered owners or holders thereof under the Indenture for any purpose.

Under the terms of the Indenture, the Issuers, the Subsidiary Guarantors and the Trustee will treat the persons in whose names the notes are registered (including notes represented by Global notes) as the owners thereof for the purpose of receiving payments and for any and all other purposes whatsoever. Payments in respect of the principal of, premium, if any, and interest on Global notes registered in the name of DTC or its nominee will be payable by the Trustee to DTC or its nominee as the registered holder under the Indenture. Consequently, none of the Issuers, the Subsidiary Guarantors, the Trustee nor any agent of the Issuers, the Subsidiary Guarantors or the Trustee has or will have any responsibility or liability for (i) any aspect of DTC's records or any Direct Participant's or Indirect Participant's records relating to or payments made on account of beneficial ownership interests in the Global notes or for maintaining, supervising or reviewing any of DTC's records or any Direct Participant's or Indirect Participant's records relating to the beneficial ownership interests in any Global note or (ii) any other matter relating to the actions and practices of DTC or any of its Direct Participants or Indirect Participants.

DTC has advised the Issuers that its current payment practice (for payments of principal, interest and the like) with respect to securities such as the notes is to credit the accounts of the relevant Direct Participants with such payment on the payment date in amounts proportionate to such Direct Participant's respective ownership interests in the Global notes as shown on DTC's records. Payments by Direct Participants and Indirect Participants to the beneficial owners of the notes will be governed by standing instructions and customary practices between them and will not be the responsibility of DTC, the Trustee, the Issuers or the Subsidiary Guarantors. None of the Issuers, the Subsidiary Guarantors or the Trustee will be liable for any delay by DTC or its Direct Participants or Indirect Participants in identifying the beneficial owners of the notes, and the Issuers, the Subsidiary Guarantors and the Trustee may conclusively rely on and will be protected in relying on instructions from DTC or its nominee as the registered owner of the notes for all purposes.

The Global notes will trade in DTC's Same-day Funds Settlement System and, therefore, transfers between Direct Participants in DTC will be effected in accordance with DTC's procedures, and will be settled in immediately available funds. Transfers between Indirect Participants (other than Indirect Participants who hold an interest in the notes through Euroclear or Clearstream) who hold an interest through a Direct Participant will be effected in accordance with the procedures of such Direct Participant but generally will settle in immediately available funds. Transfers between and among indirect Participants who hold interests in the notes through Euroclear and Clearstream will be effected in the ordinary way in accordance with their respective rules and operating procedures.

Cross-market transfers between Direct Participants in DTC, on the one hand, and Indirect Participants who hold interests in the notes through Euroclear or Clearstream, on the other hand, will be effected by Euroclear's or Clearstream's respective Nominee through DTC in accordance with DTC's rules on behalf of Euroclear or Clearstream; however, delivery of instructions relating to cross-market transactions must be made directly to Euroclear or Clearstream and within the established deadlines (Brussels time) of such systems. Indirect Participants who hold interests in the notes through Euroclear and Clearstream may not deliver instructions directly to Euroclear's and Clearstream's Nominees. Euroclear and Clearstream will, if the transaction meets their settlement requirements, deliver instructions to their respective Nominee to deliver or receive interests on Euroclear's or Clearstream's behalf in

S-58

Table of Contents

the relevant Global note in DTC, and make or receive payment in accordance with normal procedures for same-day fund settlement applicable to DTC.

Because of time zone differences, the securities accounts of an Indirect Participant who holds an interest in the notes through Euroclear or Clearstream purchasing an interest in a Global note from a Direct Participant in DTC will be credited, and any such crediting will be reported, to Euroclear or Clearstream during the European business day immediately following the settlement date of DTC in New York. Although recorded in DTC's accounting records as of DTC's settlement date in New York, Euroclear and Clearstream customers will not have access to the cash amount credited to their accounts as a result of a sale of an interest in a Global note to a DTC Participant until the European business day for Euroclear and Clearstream immediately following DTC's settlement date.

DTC has advised us that it will take any action permitted to be taken by a holder of notes only at the direction of one or more Direct Participants to whose account interests in the Global notes are credited and only in respect of such portion of the aggregate principal amount of the notes to which such Direct Participant or Direct Participants has or have given direction. However, if there is an Event of Default under the notes, DTC reserves the right to exchange Global notes (without the direction of one or more of its Direct Participants) for notes in certificated form, and to distribute such certificated forms of notes to its Direct Participants. See "— Transfers of Interest in Global Notes for Certificated Notes."

Although DTC, Euroclear and Clearstream have agreed to the foregoing procedures to facilitate transfers of interests in the Global notes among Direct Participants, including Euroclear and Clearstream, they are under no obligation to perform or to continue to perform such procedures, and such procedures may be discontinued at any time. None of the Issuers, the Subsidiary Guarantors or the Trustee shall have any responsibility for the performance by DTC, Euroclear and Clearstream or their respective Direct and Indirect Participants of their respective obligations under the rules and procedures governing any of their operations.

The information in this section concerning DTC, Euroclear and Clearstream and their book-entry systems has been obtained from sources that the Issuers believe to be reliable, but the Issuers take no responsibility for the accuracy thereof.

Transfers of Interest in Global Notes for Certificated Notes

An entire Global note may be exchanged for definitive notes in registered, certificated form without interest coupons ("Certificated notes") in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof, if (i) DTC (x) notifies the Issuers that it is unwilling or unable to continue as depositary for the Global notes or (y) has ceased to be a clearing agency registered under the Exchange Act and, in either case, the Issuers thereupon fail to appoint a successor depositary within 90 days, or (ii) there shall have occurred and be continuing an Event of Default and DTC notifies the Trustee of its decision to exchange the Global note for Certificated notes. In any such case, upon surrender by the Direct and Indirect Participants of their interests in such Global note, Certificated notes will be issued to each person that such Direct and Indirect Participants and DTC identify to the Trustee as being the beneficial owner of the related notes.

Certificated notes delivered in exchange for any beneficial interest in any Global note will be registered in the names, and issued in any approved denominations, requested by DTC on behalf of such Direct or Indirect Participants (in accordance with DTC's customary procedures).

None of the Issuers, the Subsidiary Guarantors or the Trustee will be liable for any delay by the holder of any Global note or DTC in identifying the beneficial owners of notes, and the Issuers, the Subsidiary Guarantors and the Trustee may conclusively rely on, and will be protected in relying on, instructions from the holder of the Global note or DTC for all purposes.

S-59

Table of Contents

DESCRIPTION OF OTHER INDEBTEDNESS

Amended and Restated Credit Agreement

We are party to an Amended and Restated Credit Agreement with Wells Fargo Bank, National Association as administrative agent, which we also refer to as the Credit Facility. All capitalized terms used in this description of the Amended and Restated Credit Agreement but not defined in this prospectus supplement have the meanings given to them in the Amended and Restated Credit Agreement.

The Credit Facility currently provides for lender commitments of up to $1.3 billion, with an uncommitted accordion feature of up to $500 million. We may use up to $150 million of the Credit Facility for the issuance of letters of credit and $10 million for shorter term swingline loans. The Credit Facility matures on March 20, 2019.

The borrowings under the Credit Facility bear interest at a variable interest rate, plus basis points. The variable interest rate is based either on LIBO ("LIBO Rate Loans") or the higher of (a) the prime rate set by the Credit Facility's administrative agent, (b) the Federal Funds Rate plus 0.5% and (c) the rate for LIBO for a one month interest period plus 1% ("Alternate Base Rate Loans"). The basis points correspond to the ratio of our Consolidated Funded Debt to our Adjusted Consolidated EBITDA, ranging from 0.5% to 1.5% for Alternate Base Rate Loans and from 1.5% to 2.5% for LIBO Rate Loans.

We have the right to release collateral under the Credit Facility once our long-term, senior unsecured debt has received an investment grade rating from Standard & Poor's equal to or more favorable than BBB– (stable) and from Moody's equal to or more favorable than Baa3 (stable) and our Total Leverage Ratio is not greater than 5.00 to 1.00. After such collateral release, the basis points are determined by the credit rating for our long-term, senior unsecured debt and will range from 0.125% to 1% for Alternate Base Rate Loans and from 1.125% to 2% for LIBO Rate Loans.

The Credit Facility is guaranteed by all of our wholly-owned subsidiaries (other than Liberty and its subsidiaries and MarkWest Energy West Texas Gas Company, L.L.C.) and is secured by substantially all of our assets and those of our wholly owned subsidiaries (other than Liberty and its subsidiaries).

Senior Notes

We and MarkWest Energy Finance Corporation have outstanding $500.0 million aggregate principal amount of 6.75% Senior Notes due 2020, $325.0 million aggregate principal amount of 6.5% Senior Notes due 2021, $455.0 million aggregate principal amount of 6.25% Senior Notes due 2022, $750.0 million aggregate principal amount of 5.5% Senior Notes due 2023, $1.0 billion aggregate principal amount of 4.5% Senior Notes due 2023 and approximately $1.16 billion aggregate principal amount of 4.875% Senior Notes due 2024, exclusive of discounts.

The indentures governing our outstanding series of senior notes limit our activities and the activities of our restricted subsidiaries. Each indenture places limits on our and our restricted subsidiaries' ability to: incur additional indebtedness; declare or pay dividends or distributions or redeem, repurchase or retire equity interests or subordinated indebtedness; make investments; incur liens; create any consensual limitation on the ability of our restricted subsidiaries to pay dividends, make loans or transfer property to us; engage in transactions with our affiliates; sell assets, including equity interests of our subsidiaries; make any payment on or with respect to, or purchase, redeem, defease or otherwise acquire or retire for value, any subordinated obligation or guarantor subordination obligation (except principal and interest at maturity); and consolidate, merge or transfer assets.

S-60

Table of Contents

CERTAIN U.S. FEDERAL INCOME TAX CONSEQUENCES

The following discussion summarizes certain U.S. federal income tax consequences that may be relevant to the acquisition, ownership and disposition of the notes. This discussion is based upon the provisions of the Internal Revenue Code of 1986, as amended (the "Code"), applicable U.S. Treasury Regulations promulgated thereunder, judicial authority and administrative interpretations, all as of the date of this document, and all of which are subject to change, possibly with retroactive effect, or are subject to different interpretations. We cannot assure you that the Internal Revenue Service, or IRS, will not challenge one or more of the tax consequences described in this discussion, and we have not obtained, nor do we intend to obtain, a ruling from the IRS or an opinion of counsel with respect to the U.S. federal tax consequences of acquiring, holding or disposing of the notes.

This discussion is limited to holders who purchase the notes in this offering for cash at a price equal to the issue price of the notes (i.e., the first price at which a substantial amount of the notes is sold for cash other than to bond houses, brokers or similar persons or organizations acting in the capacity of underwriters, placement agents or wholesalers) and who hold the notes as capital assets (generally, property held for investment). This discussion does not address any U.S. federal tax considerations other than U.S. federal income tax considerations (such as estate and gift tax considerations), or the tax considerations arising under the laws of any foreign, state, local or other jurisdiction or any income tax treaty. In addition, this discussion does not address all tax considerations that may be important to a particular holder in light of the holder's circumstances, or to certain categories of investors that may be subject to special rules, such as:

•: dealers in securities or currencies;
•: traders in securities that have elected the mark-to-market method of accounting for their securities;
•: U.S. holders (as defined below) whose functional currency is not the U.S. dollar;
•: persons holding notes as part of a hedge, straddle, conversion or other "synthetic security" or integrated transaction;
•: former U.S. citizens or long-term residents of the United States;
•: financial institutions;
•: insurance companies;
•: regulated investment companies;
•: real estate investment trusts;
•: persons subject to the alternative minimum tax;
•: persons tendering the 2020 Notes, the 2021 Notes and/or the 2022 Notes in the Tender Offers and concurrently purchasing the Notes;
•: entities that are tax-exempt for U.S. federal income tax purposes; and
•: partnerships and other pass-through entities and holders of interests therein.

If an entity or arrangement treated as a partnership for U.S. federal income tax purposes holds notes, the U.S. federal income tax treatment of a partner of the partnership generally will depend upon the status of the partner and the activities of the partnership and upon certain determinations made at the partner level. If you are a partner of a partnership considering an investment in the notes, you are urged to consult your own tax advisor about the U.S. federal income tax consequences of acquiring, holding and disposing of the notes.

INVESTORS CONSIDERING THE PURCHASE OF NOTES ARE URGED TO CONSULT THEIR OWN TAX ADVISORS REGARDING THE APPLICATION OF THE U.S. FEDERAL INCOME TAX LAWS TO THEIR PARTICULAR SITUATIONS AS WELL AS ANY TAX CONSEQUENCES OF THE

S-61

Table of Contents

ACQUISITION, OWNERSHIP AND DISPOSITION OF THE NOTES UNDER OTHER U.S. FEDERAL TAX LAWS OR UNDER THE LAWS OF ANY STATE, LOCAL OR FOREIGN JURISDICTION OR UNDER ANY APPLICABLE INCOME TAX TREATY.

Certain Additional Payments

In certain circumstances (see "Description of Notes — Optional Redemption" and "Description of Notes — Repurchase at the Option of Holders — Change of Control"), we may be obligated to pay amounts on the notes that are in excess of stated interest or principal on the notes. These potential payments may implicate the provisions of the U.S. Treasury Regulations relating to "contingent payment debt instruments." We do not intend to treat the possibility of paying such additional amounts as causing the notes to be treated as contingent payment debt instruments. It is possible that the IRS may take a different position, in which case, if such position is sustained, a holder might be required to accrue ordinary interest income at a higher rate than the stated interest rate and to treat as ordinary income rather than capital gain any gain realized on the taxable disposition of the notes. The remainder of this discussion assumes that the notes will not be treated as contingent payment debt instruments. You are encouraged to consult your own tax advisor regarding the possible application of the contingent payment debt instrument rules to the notes.

Tax Consequences to U.S. Holders

The following summary will apply to you if you are a U.S. holder of the notes. You are a "U.S. holder" for purposes of this discussion if you are a beneficial owner of a note and you are for U.S. federal income tax purposes:

•: an individual who is a U.S. citizen or U.S. resident alien;
•: a corporation that was created or organized in or under the laws of the United States, any state thereof or the District of Columbia;
•: an estate whose income is subject to U.S. federal income taxation regardless of its source; or
•: a trust (1) if a court within the United States is able to exercise primary supervision over the administration of the trust and one or more United States persons have the authority to control all substantial decisions of the trust, or (2) that has a valid election in effect under applicable U.S. Treasury Regulations to be treated as a United States person.

Interest on the Notes

Interest on the notes generally will be taxable to you as ordinary income at the time it is received or accrued in accordance with your regular method of accounting for U.S. federal income tax purposes.

Disposition of the Notes

You will generally recognize capital gain or loss on the sale, redemption, exchange, retirement or other taxable disposition of a note equal to the difference, if any, between the proceeds you receive (excluding any proceeds attributable to accrued but unpaid interest, which will be taxable as ordinary interest income to the extent you have not previously included such amounts in income) and your adjusted tax basis in the note. The proceeds you receive will include the amount of any cash and the fair market value of any other property received for the note. Your adjusted tax basis in the note will generally equal the amount you paid for the note. Any gain or loss will be long-term capital gain or loss if you held the note for more than one year at the time of the sale, redemption, exchange, retirement or other taxable disposition. Long-term capital gains of individuals, estates and trusts currently are eligible for reduced rates of U.S. federal income tax. The deductibility of capital losses may be subject to limitation.

S-62

Table of Contents

Information Reporting and Backup Withholding

Information reporting generally will apply to payments of interest on, and the proceeds of the sale or other disposition (including a redemption, exchange or retirement) of, notes held by you, and backup withholding will apply to such payments unless you provide to the applicable withholding agent your taxpayer identification number, certified under penalties of perjury, as well as certain other information or otherwise establish an exemption from backup withholding. Backup withholding is not an additional tax. Any amount withheld under the backup withholding rules is allowable as a credit against your U.S. federal income tax liability, if any, and a refund may be obtained from the IRS if the amounts withheld exceed your actual U.S. federal income tax liability and you timely provide the required information or appropriate claim form to the IRS.

Additional Tax on Net Investment Income

An additional 3.8% tax is imposed on the "net investment income" of certain United States citizens and resident aliens, and on the undistributed "net investment income" of certain estates and trusts. Among other items, "net investment income" generally includes gross income from interest and net gain from the disposition of property, such as the notes, less certain deductions. You are encouraged to consult your tax advisor with respect to this additional tax and its applicability in your particular circumstances.

Tax Consequences to Non-U.S. Holders

The following summary will apply to you if you are a non-U.S. holder of notes. You are a "non-U.S. holder" for purposes of this discussion if you are a beneficial owner of notes that is, for U.S. federal income tax purposes, an individual, corporation, estate or trust that is not a U.S. holder.

Interest on the Notes

Subject to the discussion of backup withholding and FATCA withholding, below, payments to you of interest on the notes generally will not be subject to U.S. federal income tax and will be exempt from withholding of U.S. federal income tax under the "portfolio interest" exemption if you properly certify as to your foreign status, as described below, and:

•: you do not own, actually or constructively, 10% or more of our capital or profits interests;
•: you are not a "controlled foreign corporation" that is related to us (actually or constructively);
•: you are not a bank whose receipt of interest on the notes is in connection with an extension of credit made pursuant to a loan agreement entered into in the ordinary course of your trade or business; and
•: interest on the notes is not effectively connected with your conduct of a U.S. trade or business.

The portfolio interest exemption generally applies only if you also appropriately certify as to your foreign status. You can generally meet the certification requirement by providing a properly executed IRS Form W-8BEN or IRS Form W-8BEN-E (or applicable successor form) to the applicable withholding agent. If you hold the notes through a financial institution or other agent acting on your behalf, you may be required to provide appropriate certifications to the agent. Your agent will then generally be required to provide appropriate certifications to the applicable withholding agent, either directly or through other intermediaries. Special rules apply to foreign partnerships, estates and trusts, and in certain circumstances certifications as to the foreign status of partners, trust owners or beneficiaries may have to be provided to the applicable withholding agent. In addition, special rules apply to qualified intermediaries that enter into withholding agreements with the IRS.

If you cannot satisfy the requirements described above, payments of interest made to you will be subject to U.S. federal withholding tax at a 30% rate, unless (i) you provide the applicable withholding agent with a properly executed IRS Form W-8BEN or IRS Form W-8BEN-E (or applicable successor

S-63

Table of Contents

form) claiming an exemption from (or a reduction of) withholding under the benefits of an income tax treaty, or (ii) the payments of such interest are effectively connected with your conduct of a trade or business in the United States (and, if required by an applicable income tax treaty, are attributable to a permanent establishment maintained by you in the United States) and you meet the certification requirements described below. (See "— Income or Gain Effectively Connected with a U.S. Trade or Business.")

The certifications described above and below must be provided to the applicable withholding agent prior to the payment of interest and must be updated periodically. If you do not timely provide the applicable withholding agent with the required certification, but you qualify for a reduced rate under an applicable income tax treaty, you may obtain a refund of any excess amounts withheld if you timely provide the required information or appropriate claim form to the IRS.

Disposition of the Notes

Subject to the discussion of backup withholding and FATCA withholding, below, you generally will not be subject to U.S. federal income tax on any gain (excluding any amount attributable to accrued and unpaid interest, which generally will be treated as interest and may be subject to the rules discussed above in "— Interest on the Notes") realized on the sale, redemption, exchange, retirement or other taxable disposition of a note unless:

•: the gain is effectively connected with the conduct by you of a U.S. trade or business (and, if required by an applicable income tax treaty, you maintain a permanent establishment in the United States to which such gain is attributable); or
•: you are a non-resident alien individual who has been present in the United States for 183 days or more in the taxable year of disposition and certain other requirements are met.

If your gain is described in the first bullet point above, you generally will be subject to U.S. federal income tax in the manner described under "— Income or Gain Effectively Connected with a U.S. Trade or Business." If you are a non-U.S. holder described in the second bullet point above, you will be subject to a flat 30% (or lower applicable income tax treaty rate) U.S. federal income tax on the gain derived from the sale or other disposition, which may be offset by certain U.S. source capital losses.

Income or Gain Effectively Connected with a U.S. Trade or Business

If any interest on the notes or gain from the sale, redemption, exchange, retirement or other taxable disposition of the notes is effectively connected with a U.S. trade or business conducted by you (and, if required by an applicable income tax treaty, you maintain a permanent establishment in the United States to which such interest or gain is attributable), then the interest income or gain will be subject to U.S. federal income tax at regular graduated income tax rates generally in the same manner as if you were a U.S. holder. Effectively connected interest income will not be subject to U.S. federal withholding tax if you satisfy certain certification requirements by providing to the applicable withholding agent a properly executed IRS Form W-8ECI (or successor form). In addition, if you are a corporation, that portion of your earnings and profits that is effectively connected with your U.S. trade or business may also be subject to a "branch profits tax" at a 30% rate, unless an applicable income tax treaty provides for a lower rate. For this purpose, interest received on a note and gain recognized on the disposition of a note will be included in earnings and profits if the interest or gain is effectively connected with the conduct by you of a U.S. trade or business.

Information Reporting and Backup Withholding

Payments to you of interest on a note, and amounts withheld from such payments, if any, generally will be required to be reported to the IRS and to you. Copies of the information returns reporting such interest payments and withholding may also be made available to the tax authorities of the country in which you reside or are established under the provisions of a specific treaty or agreement.

S-64

Table of Contents

Backup withholding generally will not apply to payments to you of interest on a note if the certification described in "— Tax Consequences to Non-U.S. Holders — Interest on the Notes" is duly provided or you otherwise establish an exemption.

Proceeds from the disposition of a note effected by the U.S. office of a U.S. or foreign broker will be subject to information reporting requirements and backup withholding unless you properly certify under penalties of perjury as to your foreign status on IRS Form W-8BEN or IRS Form W-8BEN-E (or other applicable or successor IRS Form W-8) and certain other conditions are met or you otherwise establish an exemption. Information reporting requirements and backup withholding generally will not apply to any proceeds from the disposition of a note effected outside the United States by a foreign office of a broker. However, unless such a broker has documentary evidence in its records that you are not a United States person and certain other conditions are met, or you otherwise establish an exemption, information reporting will apply to a payment of the proceeds of the disposition of a note effected outside the United States by such a broker if it has certain relationships with the United States.

Backup withholding is not an additional tax. Any amount withheld under the backup withholding rules is allowable as a credit against your U.S. federal income tax liability, if any, and a refund may be obtained from the IRS if the amounts withheld exceed your actual U.S. federal income tax liability and you timely provide the required information or appropriate claim form to the IRS.

Withholding on Payments to Certain Foreign Entities

Sections 1471 through 1474 of the Code and the U.S. Treasury Regulations and administrative guidance issued thereunder (referred to as "FATCA") impose a 30% U.S. federal withholding tax on payments of interest on the notes and on the gross proceeds from the sale or other disposition of the notes (if such sale or other disposition occurs after December 31, 2016), if paid to a "foreign financial institution" or a "non-financial foreign entity" (each as defined in the Code) (including, in some cases, when such foreign financial institution or non-financial foreign entity is acting as an intermediary), unless: (i) in the case of a foreign financial institution, such institution enters into an agreement with the U.S. government to withhold on certain payments, and to collect and provide to the U.S. tax authorities substantial information regarding U.S. account holders of such institution (which includes certain equity and debt holders of such institution, as well as certain account holders that are foreign entities with U.S. owners); (ii) in the case of a non-financial foreign entity, such entity certifies that it does not have any "substantial United States owners" (as defined in the Code) or provides the withholding agent with a certification identifying its direct and indirect substantial United States owners (generally by providing an IRS Form W-8BEN-E); or (iii) the foreign financial institution or non-financial foreign entity otherwise qualifies for an exemption from these rules and provides appropriate documentation (such as an IRS Form W-8BEN-E). Foreign financial institutions located in jurisdictions that have an intergovernmental agreement with the United States with respect to these rules may be subject to different rules. Under certain circumstances, a beneficial owner of notes might be eligible for refunds or credits of such taxes.

Under the applicable Treasury Regulations, FATCA withholding generally will apply to all U.S.-source "withholdable payments" without regard to whether the beneficial owner of the payment would otherwise be entitled to an exemption from imposition of withholding tax pursuant to an applicable income tax treaty with the United States or U.S. domestic law.

THE PRECEDING DISCUSSION OF CERTAIN U.S. FEDERAL INCOME TAX CONSEQUENCES IS FOR GENERAL INFORMATION ONLY AND IS NOT TAX ADVICE. WE URGE YOU TO CONSULT YOUR OWN TAX ADVISOR REGARDING THE PARTICULAR U.S. FEDERAL, STATE, LOCAL AND FOREIGN TAX CONSEQUENCES OF ACQUIRING, HOLDING AND DISPOSING OF OUR NOTES, INCLUDING THE CONSEQUENCES OF ANY PROPOSED CHANGE IN APPLICABLE LAWS.

S-65

Table of Contents

CERTAIN ERISA CONSIDERATIONS

The following is a summary of certain considerations associated with the purchase of the notes by employee benefit plans that are subject to Title I of the U.S. Employee Retirement Income Security Act of 1974, as amended ("ERISA"), plans, individual retirement accounts and other arrangements that are subject to Section 4975 of the Code or provisions under any federal, state, local, non-U.S. or other laws or regulations that are similar to such provisions of ERISA or the Code (collectively, "Similar Laws"), and entities whose underlying assets are considered to include "plan assets" of such plans, accounts and arrangements (each, a "Plan").

General Fiduciary Matters

ERISA and the Code impose certain duties on persons who are fiduciaries of a Plan subject to Title I of ERISA or Section 4975 of the Code (an "ERISA Plan") and prohibit certain transactions involving the assets of an ERISA Plan and its fiduciaries or other interested parties. Under ERISA and the Code, any person who exercises any discretionary authority or control over the administration of an ERISA Plan or the management or disposition of the assets of an ERISA Plan, or who renders investment advice for a fee or other compensation to an ERISA Plan, is generally considered to be a fiduciary of the ERISA Plan.

In considering an investment in the notes of a portion of the assets of any Plan, a fiduciary should determine whether the investment is in accordance with the documents and instruments governing the plan and the applicable provisions of ERISA, the Code or any Similar Law relating to a fiduciary's duties to the Plan including, without limitation, the prudence, diversification, delegation of control and prohibited transaction provisions of ERISA, the Code and any other applicable Similar Laws.

Prohibited Transaction Issues

Section 406 of ERISA and Section 4975 of the Code prohibit ERISA Plans from engaging in specified transactions involving plan assets with persons or entities who are "parties in interest," within the meaning of ERISA, or "disqualified persons," within the meaning of Section 4975 of the Code, unless an exemption is available. A party in interest or disqualified person who engaged in a non-exempt prohibited transaction may be subject to excise taxes and other penalties and liabilities under ERISA and the Code. In addition, the fiduciary of the ERISA Plan that engaged in such a non-exempt prohibited transaction may be subject to penalties and liabilities under ERISA and the Code.

The acquisition and/or holding of notes by an ERISA Plan with respect to which the Issuers, the underwriter, the Subsidiary Guarantors, the Trustee or any of their respective affiliates is considered a party in interest or a disqualified person may constitute or result in a direct or indirect prohibited transaction under Section 406 of ERISA and/or Section 4975 of the Code, unless the investment is acquired and is held in accordance with an applicable statutory, class or individual prohibited transaction exemption. In this regard, the United States Department of Labor (the "DOL") has issued prohibited transaction class exemptions, or "PTCEs," that may apply to the acquisition and holding of the notes. These class exemptions include, without limitation, PTCE 84-14 respecting transactions determined by independent qualified professional asset managers, PTCE 90-1 respecting insurance company pooled separate accounts, PTCE 91-38 respecting bank collective investment funds, PTCE 95-60 respecting life insurance company general accounts and PTCE 96-23 respecting transactions determined by in-house asset managers, although there can be no assurance that all of the conditions of any such exemptions will be satisfied. Because of the foregoing, the notes should not be purchased or held by any person investing "plan assets" of any Plan, unless such purchase and holding will not constitute a non-exempt prohibited transaction under ERISA and the Code or violation of any applicable Similar Laws.

S-66

Table of Contents

Representation

Accordingly, by its acceptance of a note, each purchaser and subsequent transferee of a note (or any interest therein) will be deemed to have represented and warranted that either (i) no portion of the assets used by such purchaser or transferee to acquire and hold the notes (or any interest therein) constitutes assets of any Plan or (ii) the purchase and holding of the notes (or any interest therein) by such purchaser or transferee will not constitute a non-exempt prohibited transaction under Section 406 of ERISA or Section 4975 of the Code or violation under any applicable Similar Laws.

The foregoing discussion is general in nature and is not intended to be all-inclusive. Due to the complexity of these rules and the penalties that may be imposed upon persons involved in nonexempt prohibited transactions, it is particularly important that fiduciaries, or other persons considering purchasing the notes on behalf of, or with the assets of, any Plan, consult with their counsel regarding the potential applicability of ERISA, Section 4975 of the Code and any Similar Laws to such investment and whether an exemption would be applicable to the purchase and holding of the notes.

S-67

Table of Contents

UNDERWRITING

Subject to the terms and conditions in the underwriting agreement dated the date of this prospectus supplement by and among us, the subsidiary guarantors and the underwriters named below, for whom Wells Fargo Securities, LLC is acting as representative, we have agreed to sell to each of the underwriters, and each of the underwriters has agreed to purchase from us, severally and not jointly, the principal amount of the notes set forth opposite the underwriter's name.

Name of Underwriter		Principal Amount of Notes

Wells Fargo Securities, LLC		$156,000,000
Barclays Capital Inc.		123,000,000
Citigroup Global Markets Inc.		72,000,000
Goldman, Sachs & Co.		72,000,000
J.P. Morgan Securities LLC		72,000,000
Merrill Lynch, Pierce, Fenner & Smith Incorporated		72,000,000
Morgan Stanley & Co. LLC		72,000,000
RBC Capital Markets, LLC		72,000,000
SunTrust Robinson Humphrey, Inc.		72,000,000
U.S. Bancorp Investments, Inc.		72,000,000
UBS Securities LLC		72,000,000
BBVA Securities Inc.		39,000,000
BNP Paribas Securities Corp.		39,000,000
Capital One Securities, Inc.		39,000,000
Comerica Securities, Inc.		39,000,000
Natixis Securities Americas LLC		39,000,000
PNC Capital Markets LLC		39,000,000
SMBC Nikko Securities America, Inc.		39,000,000

Total		$1,200,000,000

The underwriting agreement provides that the obligation of the underwriters to purchase the notes included in this offering is subject to approval of legal matters by counsel and certain other conditions. Under the terms and conditions of the underwriting agreement, if the underwriters purchase any of the notes, then they are obligated to purchase all of the notes.

The underwriters propose to offer the notes initially at the public offering price on the cover page of this prospectus supplement. After the initial offering of the notes to the public, the public offering price and other selling terms to dealers may be changed. The underwriters may offer and sell notes through certain of their affiliates.

We and the subsidiary guarantors have agreed not to offer or sell any of our debt securities (other than the notes) for a period of 90 days after the date of this prospectus supplement without the prior written consent of Wells Fargo Securities, LLC.

We estimate that the total expenses of this offering to be paid by us, excluding underwriting discounts, will be approximately $1.7 million.

In connection with this offering and in compliance with applicable law, the underwriters may engage in over-allotment, stabilizing and syndicate covering transactions and penalty bids in accordance with Regulation M under the Exchange Act.

•: Over-allotment involves syndicate sales in excess of the offering size, which creates a syndicate short position.

S-68

Table of Contents

•: The underwriters may also effect transactions which stabilize, maintain or otherwise affect the market price of the notes at levels above those which might otherwise prevail in the open market.
•: Such transactions may include placing bids for the notes or effecting purchases of the notes for the purpose of pegging, fixing or maintaining the price of the notes.
•: Syndicate covering transactions involve purchases of the notes in the open market after the distribution has been completed in order to cover syndicate short positions.
•: Penalty bids permit the underwriters to reclaim a selling concession from a syndicate member when the notes sold by that syndicate member are purchased in a syndicate covering transaction to cover syndicate short positions.

These stabilizing transactions, syndicate covering transactions and penalty bids may have the effect of preventing or retarding a decline in the market price of the notes. They may also cause the price of the notes to be higher than it would otherwise be in the absence of these transactions. These transactions may be effected in the over-the-counter market or otherwise. The underwriters are not required to engage in any of these activities and such activities, if commenced, may be discontinued at any time.

Neither we nor any of the underwriters makes any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of the notes. In addition, neither we nor any of the underwriters makes any representation that the underwriters will engage in such transactions or that such transactions, once commenced, will not be discontinued without notice.

The notes are offered for sale only in those jurisdictions where it is legal to offer them. The offering of the notes by the underwriters is subject to receipt and acceptance and subject to the underwriters' right to reject any order in whole or in part.

There is no public market for the notes. The notes will not be listed on any securities exchange or included in any automated quotation system. The underwriters have advised us that, following completion of the offering of the notes, they intend to make a market in the notes, as permitted by applicable law. The underwriters are not obligated, however, to make a market in the notes, and may discontinue any market making activities at any time without notice, in their sole discretion. If the underwriters cease to act as a market maker for the notes for any reason, there can be no assurance that another firm or person will make a market in the notes. Accordingly, we cannot assure you as to the development or liquidity of any market for the notes.

We have agreed to indemnify the several underwriters against certain liabilities, including liabilities under the Securities Act, the Exchange Act or other federal or state statutory law or to contribute to payments that the underwriters may be required to make in respect of any such liabilities.

Certain Relationships

Certain of the underwriters and their affiliates have performed investment banking, commercial banking and advisory services for us and our affiliates from time to time for which they have received customary fees and expenses. The underwriters and their affiliates may, from time to time in the future, engage in transactions with and perform services for us and our affiliates in the ordinary course of business. Certain of the underwriters or their affiliates may be holders of notes subject to the Tender Offers and may therefore also receive a portion of the proceeds from this offering in respect of such notes.

Because the Financial Industry Regulatory Authority, Inc. ("FINRA") views our notes as interests in a direct participation program, any offering of notes under the registration statement of which this prospectus forms a part will be made in compliance with Rule 2310 of the FINRA rules.

S-69

Table of Contents

Certain of the underwriters or their respective affiliates have a lending relationship with us, and as a result, certain of these underwriters or their respective affiliates routinely hedge and certain of those underwriters or their respective affiliates may hedge their credit exposure to us consistent with their customary risk management policies. Typically, these underwriters and their respective affiliates would hedge such exposure by entering into transactions which consist of either the purchase of credit default swaps or the creation of short positions in our securities, including potentially the notes offered hereby. Any such credit default swaps or short positions could adversely affect future trading prices of the notes offered hereby.

The underwriters and their respective affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of our assets, securities or instruments and the underwriters may at any time hold, or recommend to clients that they should acquire long and/or short positions of our assets, securities and instruments.

Selling Restrictions

United Kingdom

This prospectus supplement and the accompanying prospectus have not been approved by an authorized person for the purposes of section 21 of the Financial Services and Markets Act 2000 ("FSMA") and are, accordingly, only being distributed in the United Kingdom to, and are only directed at (i) investment professionals falling within the description of persons in Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005, as amended (the "Financial Promotion Order"); or (ii) high net worth companies and other persons falling within Article 49(2)(a) to (d) of the Financial Promotion Order; or (iii) to any other person to whom they may otherwise lawfully be communicated or made in accordance with the Financial Promotion Order (all such persons together being referred to as "relevant persons").

The notes are only available to, and any invitation, offer or agreement to subscribe, purchase or otherwise acquire such notes will be engaged in only with, relevant persons. Any person who is not a relevant person should not act or rely on this document or any of its contents.

An invitation or inducement to engage in investment activity (within the meaning of Section 21 of FSMA) in connection with the issue or sale of any notes which are the subject of the offering contemplated by this prospectus will only be communicated or caused to be communicated in circumstances in which Section 21(1) of FSMA does not apply to the issuers.

EEA

In relation to each Member State of the European Economic Area which has implemented the Prospectus Directive (each, a "Relevant Member State"), each underwriter has represented and agreed that with effect from and including the date on which the Prospectus Directive is implemented in that Relevant Member State (the "Relevant Implementation Date") not made and will not make an offer of notes which are the subject of the offering contemplated by this prospectus to the public in that Relevant Member State other than:

•: to any legal entity which is a qualified investor as defined in the Prospectus Directive;
•: to fewer than 100 or, if the Relevant Member State has implemented the relevant provision of the 2010 PD Amending Directive, 150, natural or legal persons (other than qualified investors as defined in the Prospectus Directive), as permitted under the Prospectus Directive, subject to obtaining the prior consent of the representatives for any such offer; or

S-70

Table of Contents

•: in any other circumstances falling within Article 3(2) of the Prospectus Directive, provided that no such offer of notes shall require us or the underwriter to publish a prospectus pursuant to Article 3 of the Prospectus Directive,

provided that no such offer of notes shall require us or the underwriters to publish a prospectus pursuant to Article 3 of the Prospectus Directive.

For the purposes of this provision, the expression an "offer of notes to the public" in relation to any notes in any Relevant Member State means the communication in any form and by any means of sufficient information on the terms of the offer and the notes to be offered so as to enable an investor to decide to purchase or subscribe for the notes, as the same may be varied in that Member State by any measure implementing the Prospectus Directive in that Member State, the expression "Prospectus Directive" means Directive 2003/71/EC (and amendments thereto, including the 2010 PD Amending Directive, to the extent implemented in the Relevant Member State), and includes any relevant implementing measure in the Relevant Member State and the expression "2010 PD Amending Directive" means Directive 2010/73/EU.

Hong Kong

Our notes may not be offered or sold in Hong Kong by means of this prospectus supplement or any other document other than to (a) professional investors as defined in the Securities and Futures Ordinance of Hong Kong (Cap. 571, Laws of Hong Kong) ("SFO") and any rules made under the SFO or (b) in other circumstances which do not result in this prospectus supplement being deemed to be a "prospectus," as defined in the Companies Ordinance of Hong Kong (Cap. 32, Laws of Hong Kong) ("CO"), or which do not constitute an offer to the public within the meaning of the CO or the SFO; and no person has issued or had in possession for the purposes of issue, or will issue or has in possession for the purposes of issue, whether in Hong Kong or elsewhere, any advertisement, invitation or document relating to our notes which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do so under the securities laws of Hong Kong) other than with respect to our notes which are or are intended to be disposed of only to persons outside Hong Kong or only to professional investors as defined in the SFO.

S-71

Table of Contents

VALIDITY OF THE NOTES

Certain legal matters with respect to the legality of the notes and certain tax matters relating to the notes will be passed upon for us by Vinson & Elkins L.L.P., Houston, Texas. Certain matters relating to the notes will be passed upon for the underwriters by Latham & Watkins LLP, Houston, Texas. Members of Vinson & Elkins L.L.P. involved in this offering own an aggregate of approximately 2,000 of our common units.

EXPERTS

The consolidated financial statements incorporated in this prospectus supplement by reference from the Partnership's annual report on Form 10-K, and the effectiveness of MarkWest Energy Partners, L.P. and subsidiaries' internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports, which are incorporated herein by reference. Such consolidated financial statements have been so incorporated in reliance upon the reports of such firm given upon their authority as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

We have filed a registration statement with the SEC under the Securities Act of 1933, as amended, that registers the offer and sale from time to time of our common units and debt securities, including the notes covered by this prospectus supplement. The registration statement, including the attached exhibits, contains additional relevant information about us. In addition, we file annual, quarterly and other reports and other information with the SEC. You may read and copy any document we file with the SEC at the SEC's Public Reference Room at 100 F Street, N.E., Room 1580, Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information on the operation of the SEC's Public Reference Room. The SEC maintains an Internet site that contains reports, proxy and information statements and other information regarding issuers that file electronically with the SEC. Our SEC filings are available on the SEC's website at http://www.sec.gov. You also can obtain information about us at the offices of the New York Stock Exchange, 20 Broad Street, New York, New York 10005.

The SEC allows us to "incorporate by reference" the information we have filed with the SEC. This means that we can disclose important information to you without actually including the specific information in this prospectus supplement or the accompanying base prospectus by referring you to other documents filed separately with the SEC. The information incorporated by reference is an important part of this prospectus supplement and the accompanying base prospectus. Information that we later provide to the SEC, and which is deemed to be "filed" with the SEC, will automatically update information previously filed with the SEC, and may replace information in this prospectus supplement and the accompanying base prospectus and information previously filed with the SEC.

We incorporate by reference in this prospectus supplement the documents listed below and any future filings we make with the SEC under Section 13(a), 13(c), 14 or 15(d) of the Exchange Act, after the date of this prospectus supplement and prior to the termination of this offering (excluding any information furnished but not filed pursuant to Item 2.02 or 7.01 on any current report on Form 8-K):

•: our Annual Report on Form 10-K for the year ended December 31, 2014, as filed with the SEC on February 25, 2015;
•: our Quarterly Report on Form 10-Q for the quarter ended March 31, 2015, as filed with the SEC on May 6, 2015;
•: our Current Reports on Form 8-K as filed with the SEC on January 23, 2015, February 3, 2015, March 3, 2015, May 5, 2015, May 19, 2015 and May 28, 2015; and
•: the information responsive to Part III of Form 10-K for the year ended December 31, 2014 provided in our proxy statement filed on April 23, 2015.

S-72

Table of Contents

These reports contain important information about us, our financial condition and our results of operations.

You may request a copy of any document incorporated by reference in this prospectus supplement and any exhibit specifically incorporated by reference in those documents, at no cost, by writing or telephoning us at the following address or phone number:

MarkWest Energy Partners, L.P.
1515 Arapahoe Street, Tower 1, Suite 1600
Denver, CO 80202-2137
Attention: Nancy K. Buese
(303) 925-9200

We also make available free of charge on our internet website at http://www.markwest.com our annual reports on Form 10-K, our quarterly reports on Form 10-Q, our current reports on Form 8-K and any amendments to those reports, as soon as reasonably practicable after we electronically file such material with, or furnish it to, the SEC. Information contained on our website is not part of this prospectus supplement or the accompanying base prospectus and does not constitute a part of this prospectus supplement or the accompanying base prospectus.

S-73

Table of Contents

PROSPECTUS

MarkWest Energy Partners, L.P.
MarkWest Energy Finance Corporation

Common Units
Debt Securities

We may from time to time offer the following securities under this prospectus:

•: common units representing limited partner interests in MarkWest Energy Partners, L.P., and
•: debt securities of MarkWest Energy Partners, L.P.

Our common units are traded on the New York Stock Exchange under the symbol "MWE." We will provide information in the prospectus supplement for the trading market for any debt securities we may offer.

MarkWest Energy Finance Corporation may act as co-issuer of the debt securities, and some or all other direct or indirect wholly-owned subsidiaries of MarkWest Energy Partners, L.P. (which we refer to as "Subsidiary Guarantors"), other than "minor" subsidiaries as such item is interpreted in securities regulation governing financial reporting for guarantors, may guarantee the debt securities.

Each time we sell securities we will provide a prospectus supplement that will contain specific information about the terms of that offering. This prospectus may not be used to consummate sales of securities unless accompanied by a prospectus supplement. The prospectus supplement may also add, update or change information contained in this prospectus. You should read this prospectus and any prospectus supplement carefully before you invest. You should also read the documents we have referred you to in the "Where You Can Find More Information" section of this prospectus for information on us and for our financial statements.

Limited partnerships are inherently different from corporations. You should carefully consider each of the factors described under "Risk Factors," which begin on page 6 of this prospectus before you make an investment in our securities.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus is October 26, 2012

Table of Contents

TABLE OF CONTENTS

		Page

About This Prospectus			1
Where You Can Find More Information			2
Forward-Looking Statements			4
About MarkWest Energy Partners			5
Risk Factors			6
Use of Proceeds			7
Ratios of Earnings to Fixed Charges			8
Description of Common Units			9
Description of Our Debt Securities			11
Cash Distribution Policy			20
Partnership Agreement			21
Material Tax Consequences			31
Investment in MarkWest Energy Partners by Employee Benefit Plans			45
Plan of Distribution			48
Validity of the Securities			50
Experts			50

You should rely only on the information contained or incorporated by reference in this prospectus or any prospectus supplement. We have not authorized anyone to provide you with information that is different. This prospectus may only be used where it is legal to sell our debt securities or common units. You should not assume that the information incorporated by reference or provided in this prospectus or any prospectus supplement is accurate as of any date other than the date on the front of each document. Our business, financial condition, results of operations, and prospects may have changed since that date.

Table of Contents

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that we and MarkWest Energy Finance Corporation have filed with the Securities and Exchange Commission ("SEC") using a "shelf' registration process. Under this shelf registration process, we may offer from time to time our common units or debt securities described in this prospectus in one or more offerings. Each time we offer securities, we will provide you with a prospectus supplement that will describe, among other things, the specific amounts and prices of the securities being offered and the terms of the offering. The prospectus supplement may also add, update or change information contained in this prospectus. Therefore, before you invest in our securities, you should read this prospectus, any attached prospectus supplements and the additional information described below under "Where You Can Find More Information."

For purposes of this prospectus, unless the context clearly indicates otherwise, "we," "us," "our" and the "Partnership" refer to MarkWest Energy Partners, L.P. and, when applicable, its consolidated subsidiaries; and "MarkWest Hydrocarbon" refers to MarkWest Hydrocarbon, Inc. and its direct and indirect consolidated subsidiaries.

Table of Contents

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and other reports and other information with the SEC. You may read and copy any document we file with the SEC at the SEC's Public Reference Room at 100 F Street, N.E., Room 1580, Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information on the operation of the SEC's Public Reference Room. The SEC maintains an Internet site that contains reports, proxy and information statements and other information regarding issuers that file electronically with the SEC. Our SEC filings are available on the SEC's website athttp://www.sec.gov. You also can obtain information about us at the offices of the New York Stock Exchange, 20 Broad Street, New York, New York 10005.

The SEC allows us to "incorporate by reference" the information we have filed with the SEC. This means that we can disclose important information to you without actually including the specific information in this prospectus by referring you to other documents filed separately with the SEC. The information incorporated by reference is an important part of this prospectus. Information that we later provide to the SEC, and which is deemed to be "filed" with the SEC, will automatically update information previously filed with the SEC, and may replace information in this prospectus and information previously filed with the SEC.

We incorporate by reference the documents listed below and any future filings we make with the SEC under Section 13 (a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended (excluding any information furnished pursuant to 2.02 or 7.01 on any current report on Form 8-K);

•: Our annual report on Form 10-K for the year ended December 31, 2011, as filed with the SEC on February 28, 2012;
•: Our quarterly reports on Form 10-Q for the quarter ended March 31, 2012, as filed with the SEC on May 7, 2012 and for the quarter ended June 30, 2012, as filed with the SEC on August 6, 2012; and
•: Our current reports on Form 8-K as filed with the SEC on January 31, 2012, February 6, 2012, March 7, 2012, March 16, 2012, April 30, 2012, May 7, 2012 (two reports), May 14, 2012, May 30, 2012, June 5, 2012, June 29, 2012, August 10, 2012 and August 17, 2012.
•: The information responsive to Part III of Form 10-K for the year ended December 31, 2011 provided in our proxy statement filed on April 20, 2012.

These reports contain important information about us, our financial condition and our results of operations.

You may request a copy of any document incorporated by reference in this prospectus and any exhibit specifically incorporated by reference in those documents, at no cost, by writing or telephoning us at the following address or phone number:

MarkWest Energy Partners, L.P.
1515 Arapahoe Street, Tower 1, Suite 1600
Denver, CO 80202
Attention: Nancy K. Buese
(303) 925-9200

We also make available free of charge on our internet website athttp://www.markwest.com our annual reports on Form 10-K, our quarterly reports on Form 10-Q, our current reports on Form 8-K and any amendments to those reports, as soon as reasonably practicable after we electronically file such material with, or furnish it to, the SEC. Information contained on our website is not part of this prospectus and does not constitute a part of this prospectus.

Table of Contents

You should rely only on the information incorporated by reference or provided in this prospectus. We have not authorized anyone else to provide you with any information. You should not assume that the information incorporated by reference or provided in this prospectus is accurate as of any date other than the date on the front of each document. We are subject to the information requirements of the Exchange Act, and in accordance therewith file reports and other information with the SEC. You may read our filings on the SEC's website and at the SEC's Public Reference Room described above. Our common stock trades on the NYSE under the symbol "MWE." Reports that we file with the NYSE may be inspected and copied at the offices of the NYSE described above.

Table of Contents

FORWARD-LOOKING STATEMENTS

Statements included or incorporated by reference in this prospectus that are not historical facts are forward-looking statements. We use words such as "could," "may," "predict," "will," "should," "expect," "hope," "continue," "potential," "plan," "intend," "anticipate," "project," "believe," "estimate," and similar expressions to identify forward-looking statements. These forward-looking statements are made based upon management's current expectations, estimates, assumptions and beliefs concerning future events impacting us and therefore involve a number of risks and uncertainties. We caution that forward-looking statements are not guarantees and that actual results could differ materially from those expressed or implied in the forward-looking statements. We do not update publicly any forward-looking statement with new information or future events. Investors are cautioned not to put undue reliance on forward-looking statements as many of these factors are beyond our ability to control or predict. Please read "Risk Factors" in Part I, Item 1A. in our most recent annual report on Form 10-K and in Part II, Item lA in our most recent quarterly reports on Form 10-Q.

Table of Contents

ABOUT MARKWEST ENERGY PARTNERS

We are a master limited partnership engaged in the gathering, transportation and processing of natural gas; the gathering, transportation, fractionation, marketing and storage of natural gas liquids, or NGLs; and the gathering and transportation of crude oil. We have extensive natural gas gathering, processing and transmission operations in the southwest, Gulf Coast and northeast regions of the United States, including the Marcellus Shale, and we are the largest natural gas processor and fractionator in the Appalachian region.

Our principal executive offices are located at 1515 Arapahoe Street, Tower 1, Suite 1600, Denver, Colorado 80202. We maintain a website athttp://www.markwest.com. The information on our website is not part of this prospectus, and you should rely only on information contained in this prospectus or incorporated herein by reference when making an investment decision.

Table of Contents

RISK FACTORS

An investment in our securities involves risks. You should carefully consider the risk factors and all of the other information included in, or incorporated by reference into, this prospectus, including those included in our most recent annual report on Form 10-K and in our most recent quarterly reports on Form 10-Q, in evaluating an investment in our securities. If any of these risks were to occur, our business, financial condition or results of operations could be adversely affected. In that case, the trading price of our common units or debt securities could decline and you could lose all or part of your investment. When we offer and sell any securities pursuant to a prospectus supplement, we may include additional risk factors relevant to such securities in the prospectus supplement.

Table of Contents

USE OF PROCEEDS

Unless we specify otherwise in any prospectus supplement, we will use the net proceeds we receive from the sale of securities covered by this prospectus for general corporate purposes, which may include, among other things:

•: paying or refinancing all or a portion of our indebtedness outstanding at the time; and
•: funding working capital, capital expenditures or acquisitions.

The actual application of proceeds from the sale of any particular offering of securities using this prospectus will be described in the applicable prospectus supplement relating to such offering. The precise amount and timing of the application of these proceeds will depend upon our funding requirements and the availability and cost of other funds.

Table of Contents

RATIOS OF EARNINGS TO FIXED CHARGES

The table below sets forth the ratios of earnings to fixed charges for us for the periods indicated.

	Year Ended December 31,

	Year Ended December 31,										Six Months Ended June 30, 2012
	2007		2008		2009		2010		2011		Six Months Ended June 30, 2012
Ratio of Earnings to Fixed Charges		(1)		4.07x		(2)		1.28x		1.98x		4.55x

(1): Earnings were inadequate to cover fixed charges for the year ended December 31, 2007 by $56,490,000
(2): Earnings were inadequate to cover fixed charges for the year ended December 31, 2009 by $169,659,000

For the six months ended June 30, 2012, our ratio of earnings to fixed charges, on a pro forma basis giving effect to our sale of $750 million aggregate principal amount of senior notes in August 7, 2012 and the use of proceeds therefrom, would have been 3.47x.

For purposes of computing the ratio of earnings to fixed charges, "earnings" is the aggregate of the following items:

•: pre-tax income or loss from continuing operations before adjustment for non-controlling interest in consolidated subsidiaries or income or loss from equity investees;
•: plus fixed charges;
•: plus distributed income of equity investees;
•: plus amortization of capitalized interest;
•: plus (less) our share pretax losses (earnings) of equity investments; and
•: less non-controlling interest in pre-tax income of subsidiaries that have not incurred fixed charges.

The term "fixed charges" means the sum of the following:

•: interest expense and capitalized interest, including the gain or loss on interest rate swaps and amortization of premiums, discounts and capitalized expenses related to indebtedness; and
•: an estimate of the interest within rental expense.

Table of Contents

DESCRIPTION OF COMMON UNITS

The Units

The common units, the Class A units and the Class B units represent limited partner interests in us. The holders of units (other than the Class B units) are generally entitled to participate in partnership distributions and exercise the rights or privileges available to limited partners under the Amended and Restated Partnership Agreement (as defined under "Partnership Agreement" below). For a description of the relative rights and preferences of holders of common units, Class A units and Class B units in and to partnership distributions, please read "Cash Distribution Policy." For a description of the rights and privileges of limited partners under the Amended and Restated Partnership Agreement, including voting rights, please read "Partnership Agreement."

Transfer Agent and Registrar

Duties

Wells Fargo Bank, N.A. serves as registrar and transfer agent for the common units. We pay all fees charged by the transfer agent for transfers of common units, except the following that must be paid by unitholders:

•: surety bond premiums to replace lost or stolen certificates, taxes and other governmental charges;
•: special charges for services requested by a holder of a common unit; and
•: other similar fees or charges.

There is no charge to unitholders for disbursements of our cash distributions. We will indemnify the transfer agent, its agents and each of their stockholders, directors, officers and employees against all claims and losses that may arise out of acts performed or omitted for its activities in that capacity, except for any liability due to any gross negligence or intentional misconduct of the indemnified person or entity.

Resignation or Removal

The transfer agent may resign, by notice to us, or be removed by us. The resignation or removal of the transfer agent will become effective upon our appointment of a successor transfer agent and registrar and its acceptance of the appointment. If no successor has been appointed and accepted the appointment within 30 days after notice of the resignation or removal, our general partner may act as the transfer agent and registrar until a successor is appointed.

Transfer of Common Units

The transfer of the common units to persons that purchase directly from the underwriters will be accomplished through the completion, execution and delivery of a transfer application by the investor. Any later transfers of a common unit will not be recorded by the transfer agent or recognized by us unless the transferee executes and delivers a transfer application. By executing and delivering a transfer application, the transferee of common units:

•: becomes the record holder of the common units and is an assignee until admitted into our partnership as a substituted limited partner;
•: automatically requests admission as a substituted limited partner in our partnership;
•: agrees to be bound by the terms and conditions of, and executes, the Amended and Restated Partnership Agreement;

Table of Contents

•: represents that the transferee has the capacity, power and authority to enter into the Amended and Restated Partnership Agreement;
•: grants powers of attorney to officers of our general partner and any liquidator of us as specified in the Amended and Restated Partnership Agreement; and
•: makes the consents and waivers contained in the Amended and Restated Partnership Agreement.

An assignee will become a substituted limited partner of our partnership for the transferred common units upon the consent of our general partner and the recording of the name of the assignee on our books and records. Our general partner may withhold its consent in its sole discretion. A transferee's broker, agent or nominee may complete, execute and deliver a transfer application. We are entitled to treat the nominee holder of a common unit as the absolute owner. In that case, the beneficial holder's rights are limited solely to those that it has against the nominee holder as a result of any agreement between the beneficial owner and the nominee holder.

Common units are securities and are transferable according to the laws governing transfer of securities. In addition to other rights acquired upon transfer, the transferor gives the transferee the right to request admission as a substituted limited partner in our partnership for the transferred common units. A purchaser or transferee of common units who does not execute and deliver a transfer application obtains only:

•: the right to assign the common unit to a purchaser or other transferee; and
•: the right to transfer the right to seek admission as a substituted limited partner in our partnership for the transferred common units.

Thus, a purchaser or transferee of common units who does not execute and deliver a transfer application:

•: will not receive cash distributions or federal income tax allocations, unless the common units are held in a nominee or "street name" account and the nominee or broker has executed and delivered a transfer application; and
•: may not receive some federal income tax information or reports furnished to record holders of common units.

The transferor of common units has a duty to provide the transferee with all information that may be necessary to transfer the common units. The transferor does not have a duty to insure the execution of the transfer application by the transferee and has no liability or responsibility if the transferee neglects or chooses not to execute and forward the transfer application to the transfer agent. Please read "Partnership Agreement—Status as Limited Partner or Assignee."

Until a common unit has been transferred on our books, we and the transfer agent may treat the record holder of the unit as the absolute owner for all purposes, except as otherwise required by law or applicable stock exchange regulations.

Table of Contents

DESCRIPTION OF OUR DEBT SECURITIES

General

The debt securities will be:

•: our direct general obligations;
•: either senior debt securities or subordinated debt securities; and
•: issued under separate indentures among us and Wells Fargo Bank, National Association, as Trustee.

MarkWest Energy Partners, L.P. may issue debt securities in one or more series, and MarkWest Energy Finance Corporation may be a co-issuer of one or more series of debt securities. MarkWest Energy Finance Corporation was incorporated under the laws of the State of Delaware in 2004, is wholly-owned by MarkWest Energy Partners, L.P., and has no material assets or any liabilities other than as a co-issuer of debt securities. Its activities will be limited to co-issuing debt securities and engaging in other activities incidental thereto. When used in this section "Description of the Debt Securities," the terms "we," "us," "our" and "issuers" refer jointly to MarkWest Energy Partners, L.P. and MarkWest Energy Finance Corporation, and the terms "MarkWest Energy Partners" and "MarkWest Finance" refer strictly to MarkWest Energy Partners, L.P. and MarkWest Energy Finance Corporation, respectively.

If we offer senior debt securities, we will issue them under a senior base indenture, dated as of November 2, 2010 among the issuers, the guarantors party thereto and Wells Fargo Bank, National Association, as trustee, which will be amended and supplemented by a supplemental indenture to create the form and terms of each series of senior debt securities that may be issued, offered and sold hereunder. If we issue subordinated debt securities, we will issue them under a subordinated base indenture, which will be amended and supplemented by a supplemental indenture to create the form and terms of each series of subordinated debt securities that may be issued, offered and sold hereunder. The senior base indenture and a form of the subordinated base indenture are filed as exhibits to the registration statement of which this prospectus is a part, and any supplemental indenture to either of them will be filed as an exhibit to a Current Report on Form 8-K in connection with the issuance of any new series of debt securities offered and sold hereunder. We refer to each base indenture, as amended and supplemented by each supplemental indenture applicable to a series of debt securities issued thereunder and offered hereby, as an "indenture." We urge you to read the relevant base and supplemental indenture because these documents, and not the summaries below, will define your rights as a holder of debt securities. Capitalized terms used in the summary have the meanings specified in the indentures.

Specific Terms of Each Series of Debt Securities in the Prospectus Supplement

A prospectus supplement and a supplemental indenture or authorizing resolutions relating to any series of debt securities being offered will include specific terms relating to the offering. These terms will include some or all of the following:

•: whether MarkWest Finance will be a co-issuer;
•: the guarantors of the debt securities, if any;
•: whether the debt securities are senior or subordinated debt securities;
•: the title of the debt securities;
•: the total principal amount of the debt securities;
•: the assets, if any, that are pledged as security for the payment of the debt securities;

Table of Contents

•: whether we will issue the debt securities in individual certificates to each holder in registered form, or in the form of temporary or permanent global securities held by a depository on behalf of holders;
•: the prices at which we will issue the debt securities;
•: the portion of the principal amount that will be payable if the maturity of the debt securities is accelerated;
•: the currency or currency unit in which the debt securities will be payable, if not U.S. dollars;
•: the dates on which the principal of the debt securities will be payable;
•: the interest rate that the debt securities will bear and the interest payment dates for the debt securities;
•: any conversion or exchange provisions;
•: any optional redemption provisions;
•: any sinking fund or other provisions that would obligate us to repurchase or otherwise redeem the debt securities; and
•: any changes to or additional events of default or covenants; and any other terms of the debt securities.

We may offer and sell debt securities, including original issue discount debt securities, at a substantial discount below their principal amount. The prospectus supplement will describe special U.S. federal income tax and any other considerations applicable to those securities. In addition, the prospectus supplement may describe certain special U.S. federal income tax or other considerations applicable to any debt securities that are denominated in a currency other than U.S. dollars.

Guarantees

If specified in the prospectus supplement respecting a series of debt securities, the subsidiaries of MarkWest Energy Partners specified in the prospectus supplement will unconditionally guarantee to each holder and the Trustee, on a joint and several basis, the full and prompt payment of principal of, premium, if any, and interest on the debt securities of that series when and as the same become due and payable, whether at stated maturity, upon redemption or repurchase, by declaration of acceleration or otherwise. If a series of debt securities is guaranteed, such series may be guaranteed by some or all of our wholly-owned subsidiaries other than "minor" subsidiaries as such term is interpreted in securities regulation governing financial reporting for guarantors. The prospectus supplement will identify the subsidiary guarantors and describe any limitation on the maximum amount of any particular guarantee and the conditions under which guarantees may be released.

The guarantees will be general obligations of the guarantors. Guarantees of subordinated debt securities will be subordinated to the Senior Indebtedness of the guarantors on the same basis as the subordinated debt securities are subordinated to the Senior Indebtedness of MarkWest Energy Partners.

Consolidation, Merger or Asset Sale

Each indenture will, in general, allow us to consolidate or merge with or into another domestic entity. It will also allow each issuer to sell, lease, transfer or otherwise dispose of all or substantially all of its assets to another domestic entity. If this happens, the remaining or acquiring entity must assume all of the issuer's responsibilities and liabilities under the indenture including the payment of all amounts due on the debt securities and performance of the issuer's covenants in the indenture.

Table of Contents

However, each indenture will impose certain requirements with respect to any consolidation or merger with or into an entity, or any sale, lease, transfer or other disposition of all or substantially all of an issuer's assets, including:

•: the remaining or acquiring entity must be organized under the laws of the United States, any state or the District of Columbia;
•: the remaining or acquiring entity must assume the issuer's obligations under the indenture; and
•: immediately after giving effect to the transaction, no Default or Event of Default (as defined under "—Events of Default and Remedies" below) may exist.

The remaining or acquiring entity will be substituted for the issuer in the indenture with the same effect as if it had been an original party to the indenture, and the issuer will be relieved from any further obligations under the indenture.

No Protection in the Event of a Change of Control

Unless otherwise set forth in the prospectus supplement, the debt securities will not contain any provisions that protect the holders of the debt securities in the event of a change of control of us or in the event of a highly leveraged transaction, whether or not such transaction results in a change of control of us.

Modification of Indentures

We may supplement or amend an indenture if the holders of a majority in aggregate principal amount of the outstanding debt securities of all series issued under the indenture affected by the supplement or amendment consent to it. Further, the holders of a majority in aggregate principal amount of the outstanding debt securities of any series may waive past defaults under the indenture and compliance by us with our covenants with respect to the debt securities of that series only. Those holders may not, however, waive any default in any payment on any debt security of that series or compliance with a provision that cannot be supplemented or amended without the consent of each holder affected. Without the consent of each outstanding debt security affected, no modification of the indenture or waiver may:

•: reduce the principal amount of debt securities whose holders must consent to an amendment, supplement or waiver;
•: reduce the principal of or change the fixed maturity of any debt security;
•: reduce or waive the premium payable upon redemption or alter or waive the provisions with respect to the redemption of the debt securities (except as may be permitted in the case of a particular series of debt securities);
•: reduce the rate of or change the time for payment of interest on any debt security;
•: waive a Default or an Event of Default in the payment of principal of or premium, if any, or interest on the debt securities (except a rescission of acceleration of the debt securities by the holders of at least a majority in aggregate principal amount of the debt securities and a waiver of the payment default that resulted from such acceleration);
•: except as otherwise permitted under the indenture, release any security that may have been granted with respect to the debt securities;
•: make any debt security payable in currency other than that stated in the debt securities;
•: in the case of any subordinated debt security, make any change in the subordination provisions that adversely affects the rights of any holder under those provisions;

Table of Contents

•: make any change in the provisions of the indenture relating to waivers of past Defaults or the rights of holders of debt securities to receive payments of principal of or premium, if any, or interest on the debt securities;
•: waive a redemption payment with respect to any debt security (except as may be permitted in the case of a particular series of debt securities);
•: except as otherwise permitted in the indenture, release any guarantor from its obligations under its guarantee or the indenture or change any guarantee in any manner that would adversely affect the rights of holders; or
•: make any change in the preceding amendment, supplement and waiver provisions (except to increase any percentage set forth therein).

We may supplement or amend an indenture without the consent of any holders of the debt securities in certain circumstances, including:

•: to establish the form of terms of any series of debt securities;
•: to cure any ambiguity, defect or inconsistency;
•: to provide for the assumption of an issuer's or guarantor's obligations to holders of debt securities in the case of a merger or consolidation or disposition of all or substantially all of such issuer's or guarantor's assets;
•: in the case of any subordinated debt security, to make any change in the subordination provisions that limits or terminates the benefits applicable to any holder of Senior Indebtedness of MarkWest Energy Partners;
•: to add or release guarantors pursuant to the terms of the indenture;
•: to make any changes that would provide any additional rights or benefits to the holders of debt securities or that do not, taken as a whole, adversely affect the rights under the indenture of any holder of debt securities;
•: to evidence or provide for the acceptance of appointment under the indenture of a successor Trustee;
•: to add any additional Events of Default; or
•: to secure the debt securities and/or the guarantees.

Events of Default and Remedies

"Event of Default," when used in an indenture, will mean any of the following with respect to the debt securities of any series:

•: failure to pay when due the principal of or any premium on any debt security of that series;
•: failure to pay, within 60 days of the due date, interest on any debt security of that series;
•: failure to pay when due any sinking fund payment with respect to any debt securities of that series;
•: failure on the part of the issuers to comply with the covenant described under "—Consolidation, Merger or Asset Sale";
•: failure to perform any other covenant in the indenture that continues for 30 days after written notice is given to the issuers;
•: certain events of bankruptcy, insolvency or reorganization of an issuer; or

Table of Contents

•: any other Event of Default provided under the terms of the debt securities of that series.

An Event of Default for a particular series of debt securities will not necessarily constitute an Event of Default for any other series of debt securities issued under an indenture. The Trustee may withhold notice to the holders of debt securities of any default (except in the payment of principal, premium, if any, or interest) if it considers such withholding of notice to be in the best interests of the holders.

If an Event of Default for any series of debt securities occurs and continues, the Trustee or the holders of at least 25% in aggregate principal amount of the debt securities of the series may declare the entire principal of, and accrued interest on, all the debt securities of that series to be due and payable immediately. If this happens, subject to certain conditions, the holders of a majority in the aggregate principal amount of the debt securities of that series can rescind the declaration.

Other than its duties in case of a default, a Trustee is not obligated to exercise any of its rights or powers under either indenture at the request, order or direction of any holders, unless the holders offer the Trustee reasonable security or indemnity. If they provide this reasonable security or indemnification, the holders of a majority in aggregate principal amount of any series of debt securities may direct the time, method and place of conducting any proceeding or any remedy available to the Trustee, or exercising any power conferred upon the Trustee, for that series of debt securities.

No Limit on Amount of Debt Securities

Neither indenture will limit the amount of debt securities that we may issue, unless we indicate otherwise in a prospectus supplement. Each indenture will allow us to issue debt securities of any series up to the aggregate principal amount that we authorize.

Registration of Notes

We will issue debt securities of a series only in registered form, without coupons, unless otherwise indicated in the prospectus supplement.

Minimum Denominations

Unless the prospectus supplement states otherwise, the debt securities will be issued only in principal amounts of $1,000 each or integral multiples of $1,000.

No Personal Liability

None of the past, present or future partners, incorporators, managers, members, directors, officers, employees, unitholders or stockholders of either issuer, the general partner of MarkWest Energy Partners or any guarantor will have any liability for the obligations of the issuers or any guarantors under either indenture or the debt securities or for any claim based on such obligations or their creation. Each holder of debt securities by accepting a debt security waives and releases all such liability. The waiver and release are part of the consideration for the issuance of the debt securities. The waiver may not be effective under federal securities laws, however.

Payment and Transfer

The Trustee will initially act as paying agent and registrar under each indenture. The issuers may change the paying agent or registrar without prior notice to the holders of debt securities, and the issuers or any of their subsidiaries may act as paying agent or registrar.

If a holder of debt securities has given wire transfer instructions to the issuers, the issuers will make all payments on the debt securities in accordance with those instructions. All other payments on

Table of Contents

the debt securities will be made at the corporate trust office of the Trustee, unless the issuers elect to make interest payments by check mailed to the holders at their addresses set forth in the debt security register.

The Trustee and any paying agent will repay to us upon request any funds held by them for payments on the debt securities that remain unclaimed for two years after the date upon which that payment has become due. After payment to us, holders entitled to the money must look to us for payment as general creditors.

Exchange, Registration and Transfer

Debt securities of any series will be exchangeable for other debt securities of the same series, the same total principal amount and the same terms but in different authorized denominations in accordance with the indenture. Holders may present debt securities for exchange or registration of transfer at the office of the registrar. The registrar will effect the transfer or exchange when it is satisfied with the documents of title and identity of the person making the request. We will not charge a service charge for any registration of transfer or exchange of the debt securities. We may, however, require the payment of any tax or other governmental charge payable for that registration.

We will not be required:

•: to issue, register the transfer of, or exchange debt securities of a series either during a period beginning 15 business days prior to the selection of debt securities of that series for redemption and ending on the close of business on the day of mailing of the relevant notice of redemption or repurchase, or between a record date and the next succeeding interest payment date; or
•: to register the transfer of or exchange any debt security called for redemption or repurchase, except the unredeemed portion of any debt security we are redeeming or repurchasing in part.

Provisions Relating only to the Senior Debt Securities

The senior debt securities will rank equally in right of payment with all of our other senior and unsubordinated debt. The senior debt securities will be effectively subordinated, however, to all of our secured debt to the extent of the value of the collateral for that debt. We will disclose the amount of our secured debt in the prospectus supplement.

Provisions Relating only to the Subordinated Debt Securities

Subordinated Debt Securities Subordinated to Senior Indebtedness

The subordinated debt securities will rank junior in right of payment to all of the Senior Indebtedness of MarkWest Energy Partners. "Senior Indebtedness" will be defined in a supplemental indenture or authorizing resolutions respecting any issuance of a series of subordinated debt securities, and the definition will be set forth in the prospectus supplement.

Payment Blockages

The subordinated indenture will provide that no payment of principal, interest and any premium on the subordinated debt securities may be made in the event:

•: we or our property is involved in any voluntary or involuntary liquidation or bankruptcy;
•: we fail to pay the principal, interest, any premium or any other amounts on any Senior Indebtedness of MarkWest Energy Partners within any applicable grace period or the maturity of such Senior Indebtedness is accelerated following any other default, subject to certain limited exceptions set forth in the subordinated indenture; or

Table of Contents

•: any other default on any Senior Indebtedness of MarkWest Energy Partners occurs that permits immediate acceleration of its maturity, in which case a payment blockage on the subordinated debt securities will be imposed for a maximum of 179 days at any one time.

No Limitation on Amount of Senior Debt

The subordinated indenture will not limit the amount of Senior Indebtedness that MarkWest Energy Partners may incur, unless otherwise indicated in the prospectus supplement.

Book Entry, Delivery and Form

The debt securities of a particular series may be issued in whole or in part in the form of one or more global certificates that will be deposited with the Trustee as custodian for The Depository Trust Company, New York, New York ("DTC"), and registered in the name of DTC's nominee, Cede & Co. This means that we will not issue certificates to each holder. Instead, one or more global debt securities will be issued to DTC, who will keep a computerized record of its participants (for example, your broker) whose clients have purchased the debt securities. The participant will then keep a record of its clients who purchased the debt securities. Unless it is exchanged in whole or in part for a certificated debt security, a global debt security may not be transferred, except that DTC, its nominees and their successors may transfer a global debt security as a whole to one another.

Beneficial interests in global debt securities will be shown on, and transfers of beneficial interests in global debt securities will be made only through, records maintained by DTC and its participants.

DTC has provided us the following information: DTC is a limited-purpose trust company organized under the New York Banking Law, a "banking organization" within the meaning of the New York Banking Law, a member of the United States Federal Reserve System, a "clearing corporation" within the meaning of the New York Uniform Commercial Code and a "clearing agency" registered under the provisions of Section 17A of the Securities Exchange Act of 1934. DTC holds securities that its participants ("Direct Participants") deposit with DTC. DTC also records the settlement among Direct Participants of securities transactions, such as transfers and pledges, in deposited securities through computerized records for Direct Participants' accounts. This eliminates the need to exchange certificates. Direct Participants include securities brokers and dealers, banks, trust companies, clearing corporations and certain other organizations.

DTC's book-entry system is also used by other organizations such as securities brokers and dealers, banks and trust companies that work through a Direct Participant. The rules that apply to DTC and its Direct Participants are on file with the SEC.

We will wire all payments on the global debt securities to DTC's nominee. We and the Trustee will treat DTC's nominee as the owner of the global debt securities for all purposes. Accordingly, we, the Trustee and any paying agent will have no direct responsibility or liability to pay amounts due on the global debt securities to owners of beneficial interests in the global debt securities.

It is DTC's current practice, upon receipt of any payment on the global debt securities, to credit Direct Participants' accounts on the payment date according to their respective holdings of beneficial interests in the global debt securities as shown on DTC's records. In addition, it is DTC's current practice to assign any consenting or voting rights to Direct Participants whose accounts are credited with debt securities on a record date, by using an omnibus proxy. Payments by Direct Participants to owners of beneficial interests in the global debt securities, and voting by Direct Participants, will be governed by the customary practices between the Direct Participants and owners of beneficial interests, as is the case with debt securities held for the account of customers registered in "street name." However, payments will be the responsibility of the Direct Participants and not of DTC, the Trustee or us.

Table of Contents

Debt securities represented by a global debt security will be exchangeable for certificated debt securities with the same terms in authorized denominations only if:

•: DTC notifies us that it is unwilling or unable to continue as depositary or if DTC ceases to be a clearing agency registered under applicable law and in either event a successor depositary is not appointed by us within 90 days; or
•: we determine not to require all of the debt securities of a series to be represented by a global debt security.

Satisfaction and Discharge; Defeasance

Each indenture will be discharged and will cease to be of further effect as to all outstanding debt securities of any series issued thereunder, when:

(a)

either:

(1): all outstanding debt securities of that series that have been authenticated (except lost, stolen or destroyed debt securities that have been replaced or paid and debt securities for whose payment money has theretofore been deposited in trust and thereafter repaid to us) have been delivered to the Trustee for cancellation; or
(2): all outstanding debt securities of that series that have not been delivered to the Trustee for cancellation have become due and payable by reason of the giving of a notice of redemption or otherwise or will become due and payable at their stated maturity within one year or are to be called for redemption within one year under arrangements satisfactory to the Trustee and in any case we have irrevocably deposited or caused to be irrevocably deposited with the Trustee as trust funds in trust cash in U.S. dollars, non-callable U.S. Government Obligations or a combination thereof, in such amounts as will be sufficient, without consideration of any reinvestment of interest, to pay and discharge the entire indebtedness of such debt securities not delivered to the Trustee for cancellation, for principal, premium, if any, and accrued interest to the date of such deposit (in the case of debt securities that have been due and payable) or the stated maturity or redemption date;

(b)

we have paid or caused to be paid all other sums payable by us under the indenture; and

(c)

we have delivered an officers' certificate and an opinion of counsel to the Trustee stating that all conditions precedent to satisfaction and discharge have been satisfied.

The debt securities of a particular series will be subject to legal or covenant defeasance to the extent, and upon the terms and conditions, set forth in the prospectus supplement.

The Trustee

Wells Fargo Bank, National Association will be the initial Trustee under each indenture. We maintain a banking relationship in the ordinary course of business with Wells Fargo Bank, National Association and some of its affiliates.

Limitations on Trustee if it is a Creditor

Each indenture will limit the right of the Trustee thereunder, in the event that it becomes a creditor of an issuer or guarantor, to obtain payment of claims in certain cases, or to realize on certain property received in respect of any such claim as security or otherwise.

Table of Contents

Certificates and Opinions to be Furnished to Trustee

Each indenture will provide that, in addition to other certificates or opinions that may be specifically required by other provisions of the indenture, every application by us for action by the Trustee must be accompanied by a certificate of certain of our officers and an opinion of counsel (who may be our counsel) stating that, in the opinion of the signers, all conditions precedent to such action have been complied with by us.

Governing Law

Each indenture and all of the debt securities will be governed by the laws of the State of New York.

Table of Contents

CASH DISTRIBUTION POLICY

Distributions of Available Cash

General. Within 45 days after the end of each quarter, we distribute all of our available cash to our unitholders of record on the applicable record date. We will make distributions of available cash to all common unitholders and Class A unitholders, pro rata and we will make distributions of Hydrocarbon Available Cash (as defined below) pro rata to our common unitholders. Class B unitholders are not entitled to participate in our distributions of available cash or of Hydrocarbon Available Cash prior to their conversion into common units. For a description of the conversion of Class B units to common units, please read "Partnership Agreement—Partnership Class B Units."

Definition of Available Cash. We define available cash in the Amended and Restated Partnership Agreement, and it generally means, for each fiscal quarter:

•

all cash and cash equivalents on hand at the end of the quarter (excluding cash at MarkWest Hydrocarbon);

•

less the amount of cash that our general partner determines in its reasonable discretion is necessary or appropriate to:

•: provide for the proper conduct of our business;
•: comply with applicable law, any of our debt instruments, or other agreements; or
•: provide funds for distributions to our unitholders for any one or more of the next four quarters;

•

plus all cash and cash equivalents on hand on the date of determination of available cash for the quarter resulting from working capital borrowings made after the end of the quarter. Working capital borrowings are generally borrowings that are made under our revolving credit facility and in all cases are used solely for working capital purposes or to pay distributions to partners.

Hydrocarbon Available Cash. Generally, Hydrocarbon Available Cash is defined as all cash and cash equivalents on hand derived from or attributable to our ownership of, or sale or other disposition of, the shares of common stock of MarkWest Hydrocarbon.

Contractual Restrictions on Our Ability to Distribute Available Cash. Our ability to distribute available cash is contractually restricted by the terms of our credit agreement. Our credit agreement contains covenants requiring us to maintain certain financial ratios and a minimum net worth. We are prohibited from making any distribution to unitholders if such distribution would cause an event of default or otherwise violate a covenant under our credit agreement or our indentures governing our outstanding notes. There is no guarantee that we will pay a quarterly distribution.

Distributions of Cash Upon Liquidation

If we dissolve in accordance with the Amended and Restated Partnership Agreement, we will sell or otherwise dispose of our assets in a process called liquidation. We will first apply the proceeds of liquidation to the payment of our creditors. We will distribute any remaining proceeds to our unitholders, including the holders of Class B units that convert to common units upon liquidation, in accordance with their capital account balances, as adjusted to reflect any gain or loss upon the sale or other disposition of our assets in liquidation.

Table of Contents

PARTNERSHIP AGREEMENT

The following is a summary of the material provisions of the Third Amended and Restated Partnership Agreement dated February 21, 2008, as amended by the Amendment No. 1 dated December 29, 2011, which is referred to herein as the "Amended and Restated Partnership Agreement."

We summarize the following provisions of the Amended and Restated Partnership Agreement elsewhere in this prospectus:

•: with regard to cash available for distribution, please read "Cash Distribution Policy."
•: with regard to the transfer of Partnership common units, please read "Description of the Common Units—Transfer of Common Units."
•: with regard to allocations of taxable income and taxable loss, please read "Material Tax Consequences."

Organization and Duration

We were organized on January 25, 2002 and will have a perpetual existence, unless dissolved in accordance with Article XII of the Amended and Restated Partnership Agreement.

Purpose

Our purpose under the Amended and Restated Partnership Agreement is limited to serving as a member of our operating company, MarkWest Energy Operating Company, L.L.C., which we refer to as "Opco," and engaging in any business activities that may be engaged in by Opco or that are approved by MarkWest Energy GP, L.L.C. (the "General Partner"). All of our operations are conducted through Opco and its subsidiaries or MarkWest Hydrocarbon. We own 100% of the outstanding membership interest of Opco and 100% of the outstanding common stock of MarkWest Hydrocarbon. The limited liability company agreement of Opco provides that Opco may, directly or indirectly, engage in:

•: its operations as conducted immediately before our initial public offering;
•: any other activity approved by the General Partner but only to the extent that the General Partner reasonably determines that, as of the date of the acquisition or commencement of the activity, the activity generates "qualifying income" as this term is defined in Section 7704 of the U.S. Internal Revenue Code of 1986, as amended (the "Code"); or
•: any activity that enhances the operations of an activity that is described in either of the two preceding clauses or any other activity provided such activity does not affect our treatment as a partnership for federal income tax purposes.

The General Partner is authorized in general to perform all acts deemed necessary to carry out our purposes and to conduct our business.

Power of Attorney

Each limited partner, and each person who acquires one of our common units from one of our unitholders and executes and delivers a transfer application, grants to the General Partner and, if appointed, a liquidator, a power of attorney to, among other things, execute and file documents required for our qualification, continuance or dissolution. The power of attorney also grants the General Partner the authority to amend, and to make consents and waivers under, the Amended and Restated Partnership Agreement subject to the terms thereof.

Table of Contents

Capital Contributions

Our unitholders are not obligated to make additional capital contributions, except as described below under "—Limited Liability."

Partnership Class A Units

Class A units represent limited partner interests in us and have identical rights and obligations of our common units except that Class A units (a) do not have the right to vote on, approve or disapprove, or otherwise consent to or not consent to any matter (including mergers, share exchanges and similar statutory authorizations) except as otherwise required by any non-waivable provision of law and (b) do not share in any cash and cash equivalents on hand, income, gains, losses, deductions and credits that are derived from or attributable to our ownership of, or sale or disposition of, the shares of MarkWest Hydrocarbon common stock.

Partnership Class B Units

Class B units represent limited partner interests in us that will convert into common units in five equal annual installments beginning on July 1, 2013. Further, prior to the conversion of the Class B units into common units, the Class B units are not entitled to vote on any matters on which the holders of common units are entitled to vote, other than those matters that disproportionately and adversely affect the rights and preferences of the Class B units in relation to other classes of interests in the Partnership. In such cases, the holders of Class B units are entitled to vote together as a single class. Class B units are not entitled to participate in any distributions of available cash prior to their conversion.

Limited Liability

Assuming that a limited partner does not participate in the control of our business within the meaning of the Delaware Act and that he otherwise acts in conformity with the provisions of the Amended and Restated Partnership Agreement, his liability under the Delaware Act will be limited, subject to possible exceptions, to the amount of capital he is obligated to contribute to us for his common units in us plus his share of any undistributed profits and assets. If it were determined, however, that the right, or exercise of the right, by the limited partners as a group:

•: to approve some amendments to the Amended and Restated Partnership Agreement; or
•: to take other action under the Amended and Restated Partnership Agreement

constituted "participation in the control" of our business for the purposes of the Delaware Act, then the limited partners could be held personally liable for our obligations under the laws of Delaware, to the same extent as the General Partner. This liability would extend to persons who transact business with us who reasonably believe that a limited partner is a general partner based on the limited partner's conduct. Neither the Amended and Restated Partnership Agreement nor the Delaware Act specifically provides for legal recourse against the General Partner if a limited partner were to lose limited liability through any fault of the General Partner. While this does not mean that a limited partner could not seek legal recourse, we know of no precedent for this type of a claim in Delaware case law.

Under the Delaware Act, a limited partnership may not make a distribution to a partner if, after the distribution, all liabilities of the limited partnership, other than liabilities to partners on account of their partnership interests and liabilities for which the recourse of creditors is limited to specific property of us, would exceed the fair value of the assets of the limited partnership. For the purpose of determining the fair value of the assets of a limited partnership, the Delaware Act provides that the fair value of property subject to liability for which recourse of creditors is limited will be included in

Table of Contents

the assets of the limited partnership only to the extent that the fair value of that property exceeds the nonrecourse liability. The Delaware Act provides that a limited partner who receives a distribution and knew at the time of the distribution that the distribution was in violation of the Delaware Act will be liable to the limited partnership for the amount of the distribution for three years from the date of distribution. Under the Delaware Act, an assignee who becomes a substituted limited partner of a limited partnership is liable for the obligations of his assignor to make contributions to us, except the assignee is not obligated for liabilities unknown to him at the time he became a limited partner and that could not be ascertained from the Amended and Restated Partnership Agreement.

Our subsidiaries conduct business in multiple states. Maintenance of our limited liability as a member of Opco may require compliance with legal requirements in the jurisdictions in which Opco conducts business, including qualifying our subsidiaries to do business there. Limitations on the liability of members for the obligations of a limited liability company have not been clearly established in many jurisdictions. If, by virtue of our membership interest in Opco or otherwise, it were determined that we were conducting business in any state without compliance with the applicable limited partnership or limited liability company statute, or that the right or exercise of the right by the limited partners as a group to remove or replace the General Partner, to approve some amendments to the Amended and Restated Partnership Agreement, or to take other action under the Amended and Restated Partnership Agreement constituted "participation in the control" of our business for purposes of the statutes of any relevant jurisdiction, then the limited partners could be held personally liable for our obligations under the law of that jurisdiction to the same extent as the General Partner under the circumstances. We will operate in a manner that the General Partner considers reasonable and necessary or appropriate to preserve the limited liability of the limited partners.

Voting Rights

The following matters specified below require a vote of our common unitholders.


Election of directors of the General Partner		Plurality of the votes cast at meetings of the limited partners.
Amendment of the Amended and Restated Partnership Agreement		Certain amendments may be made by the General Partner without the approval of our unitholders. Other amendments generally require the approval of a majority of our outstanding common units. Please read "—Amendment of the Amended and Restated Partnership Agreement."
Merger or the sale of all or substantially all of our assets		Majority of our outstanding common units. Please read "—Merger, Sale or Other Disposition of Assets."
Dissolution of us		Majority of our outstanding common units. Please read "—Termination and Dissolution."
Amendment of the limited liability company agreement and other action taken by us as sole member of Opco		Majority of our outstanding common units if such amendment or other action would adversely affect our limited partners (or any particular class of limited partners) in any material respect. Please read "—Action Relating to the Operating Company."

Additionally, we are prohibited from taking action that would disproportionately and adversely affect the rights and preferences of the Class B units in relation to other classes of interests in the Partnership without the consent of the holders of the Class B units.

Table of Contents

Issuance of Additional Securities

The Amended and Restated Partnership Agreement authorizes us to issue an unlimited number of additional partnership securities and rights to buy partnership securities for the consideration and on the terms and conditions established by the General Partner in its sole discretion without the approval of our unitholders.

It is possible that we will fund acquisitions through the issuance of additional common units in us or other equity securities. Holders of any additional common units in us that we issue will be entitled to share equally with the then-existing holders of our common units in distributions of available cash. In addition, the issuance of additional partnership interests may dilute the value of the interests of the then-existing holders of our common units in our net assets.

In accordance with Delaware law and the provisions of the Amended and Restated Partnership Agreement, we may also issue additional partnership interests that, in the sole discretion of the General Partner, have special voting rights to which holders of our common units are not entitled.

Amendment of the Amended and Restated Partnership Agreement

General. Amendments to the Amended and Restated Partnership Agreement may be proposed only by or with the consent of the General Partner, which consent may be given or withheld in its sole discretion, except as discussed below. In order to adopt a proposed amendment, other than the amendments discussed below, the General Partner must seek written approval of the holders of the number of our common units required to approve the amendment or call a meeting of the limited partners to consider and vote upon the proposed amendment. Except as described below, an amendment must be approved by a majority of our outstanding common units.

Prohibited Amendments. No amendment may be made that would:

•: enlarge the obligations of any limited partner without its consent, unless approved by at least a majority of the type or class of limited partner interests so affected;
•: enlarge the obligations of, restrict in any way any action by or rights of, or reduce in any way the amounts distributable, reimbursable or otherwise payable by us to the General Partner or any of its affiliates without the consent of the General Partner, which may be given or withheld in its sole discretion;
•: change the term of the Partnership;
•: provide that we are not dissolved upon an election to dissolve us by the General Partner that is approved by a majority of outstanding common units; or
•: give any person the right to dissolve us other than the General Partner's right to dissolve us with the approval of a majority of our outstanding common units.

The provision of the Amended and Restated Partnership Agreement preventing the amendments having the effects described in any of the clauses above can be amended upon the approval of the holders of at least 90% of our outstanding common units voting together as a single class.

No Partnership Unitholder Approval. The General Partner may generally make amendments to the Amended and Restated Partnership Agreement without the approval of any limited partner or assignee to reflect:

•: a change in our name, the location of our principal place of business, our registered agent or our registered office;
•: the admission, substitution, withdrawal, or removal of partners, as applicable, in accordance with the Amended and Restated Partnership Agreement;

Table of Contents

•: a change that, in the sole discretion of the General Partner, is necessary or advisable for us to qualify or to continue our qualification as a limited partnership or a partnership in which the limited partners have limited liability under the laws of any state or to ensure that none of us, Opco and its subsidiaries will be treated as an association taxable as a corporation or otherwise taxed as an entity for federal income tax purposes;
•: an amendment that is necessary, in the opinion of our counsel, to prevent us or the General Partner or its directors, officers, agents, or trustees from in any manner being subjected to the provisions of the Investment Company Act of 1940, the Investment Advisers Act of 1940, or plan asset regulations adopted under the Employee Retirement Income Security Act of 1974, whether or not substantially similar to plan asset regulations currently applied or proposed;
•: subject to the limitations on the issuance of additional partnership securities described above, an amendment that in the discretion of the General Partner is necessary or advisable for the authorization of additional partnership securities or rights to acquire partnership securities;
•: any amendment expressly permitted in the Amended and Restated Partnership Agreement to be made by the General Partner acting alone;
•: an amendment effected, necessitated or contemplated by a redemption and merger agreement that has been approved in accordance with the terms of the Amended and Restated Partnership Agreement;
•: any amendment that, in the discretion of the General Partner, is necessary or advisable for the formation by us of, or our investment in, any corporation, partnership or other entity, as otherwise permitted by the Amended and Restated Partnership Agreement;
•: a merger or conveyance the sole purpose of which is to effect a change in the legal form of us to another limited liability entity;
•: a change in our fiscal year or taxable year and related changes; or
•: any other amendments substantially similar to any of the matters described in the clauses above.

In addition, the General Partner may make amendments to the Amended and Restated Partnership Agreement without the approval of any limited partner or assignee if those amendments, in the discretion of the general partner:

•: do not adversely affect the limited partners (or any particular class of limited partners) in any material respect;
•: are necessary or advisable to satisfy any requirements, conditions or guidelines contained in any opinion, directive, order, ruling or regulation of any federal or state agency or judicial authority or contained in any federal or state statute;
•: are necessary or advisable to facilitate the trading of limited partner interests or to comply with any rule, regulation, guideline or requirement of any securities exchange on which the limited partner interests are or will be listed for trading, compliance with any of which the General Partner deems to be in our best interest and the best interest of the limited partners;
•: are necessary or advisable for any action taken by the General Partner relating to splits or combinations of units under the provisions of the Amended and Restated Partnership Agreement; or
•: are required to effect the intent of the provisions of the Amended and Restated Partnership Agreement or are otherwise contemplated by the Amended and Restated Partnership Agreement.

Table of Contents

Opinion of Counsel and Partnership Unitholder Approval. The General Partner will not be required to obtain an opinion of counsel that an amendment will not result in a loss of limited liability to the limited partners or result in us being treated as an entity for federal income tax purposes if one of the amendments described above under "—No Partnership Unitholder Approval" should occur. No other amendments to the Amended and Restated Partnership Agreement will become effective without the approval of holders of at least 90% of the outstanding Partnership common units unless we obtain an opinion of counsel to the effect that the amendment will not affect the limited liability under applicable law of any of our limited partners or cause us, Opco or its subsidiaries to be taxable as a corporation or otherwise to be taxed as an entity for federal income tax purposes (to the extent not previously taxed as such).

Any amendment that would have a material adverse effect on the rights or preferences of any type or class of outstanding units in relation to other classes of units will require the approval of at least a majority of the type or class of units so affected. Any amendment that reduces the voting percentage required to take any action must be approved by the affirmative vote of limited partners constituting not less than the voting requirement sought to be reduced.

Amendments requiring 80% of the Outstanding Partnership Common Units. Any amendments regarding provisions dealing with amendments to the Amended and Restated Partnership Agreement, limited partner meetings and related to election of the directors of the General Partner board at annual meetings, quorum, action without a meeting and voting rights cannot be amended without 80% of our outstanding common units voting in favor of such amendments.

Action Relating to the Operating Company

Without the approval of the holders of a majority of our outstanding common units, the General Partner is prohibited from consenting on our behalf, as the sole member of Opco, to any amendment to the limited liability company agreement of Opco or taking any action on our behalf permitted to be taken by a member of Opco in each case that would adversely affect our limited partners (or any particular class of limited partners) in any material respect.

Merger, Sale or Other Disposition of Assets

The Amended and Restated Partnership Agreement generally prohibits the General Partner, without the prior approval of the holders of a majority of our outstanding common units, from causing us to, among other things, sell, exchange or otherwise dispose of all or substantially all of our assets in a single transaction or a series of related transactions, including by way of merger, consolidation or other combination, or approving on our behalf the sale, exchange or other disposition of all or substantially all of the assets of our subsidiaries as a whole. The General Partner may, however, mortgage, pledge, hypothecate or grant a security interest in all or substantially all of our assets without that approval. The General Partner may also sell all or substantially all of our assets under a foreclosure or other realization upon those encumbrances without such approval.

If conditions specified in the Amended and Restated Partnership Agreement are satisfied, the General Partner may merge us or any of our subsidiaries into, or convey some or all of our assets to, a newly formed entity if the sole purpose of that merger or conveyance is to change our legal form into another limited liability entity. Our unitholders are not entitled to appraisal rights under the Amended and Restated Partnership Agreement or applicable Delaware law in the event of a merger or consolidation, a sale of substantially all of our assets or any other transaction or event for such purpose.

Table of Contents

Termination and Dissolution

We will continue as a limited partnership until terminated under the Amended and Restated Partnership Agreement. We will dissolve upon:

•: the election of the General Partner to dissolve us, if approved by the holders of a majority of our outstanding common units;
•: the sale, exchange or other disposition of all or substantially all of our assets and properties and our subsidiaries;
•: the entry of a decree of judicial dissolution of us; or
•: the withdrawal of the General Partner in violation of the Amended and Restated Partnership Agreement and the successor general partner is not admitted.

Liquidation and Distribution of Proceeds

Upon our dissolution, the liquidator authorized to wind up our affairs will, acting with all of the powers of the General Partner that the liquidator deems necessary or desirable in its judgment, liquidate our assets and apply the proceeds of the liquidation as provided in "Cash Distribution Policy—Distributions of Cash Upon Liquidation." The liquidator may defer liquidation of our assets for a reasonable period or distribute assets to partners in kind if it determines that a sale would be impractical or would cause undue loss to the partners.

No Withdrawal or Removal of the General Partner

The General Partner cannot withdraw and cannot be removed at any time for any reason. Any attempt of withdrawal or removal of the General Partner will be null and void. Nonetheless, if the General Partner withdraws in violation of the Amended and Restated Partnership Agreement:

•: the withdrawing General Partner must give 90 days' notice to the limited partners;
•: the successor general partner will be elected by a plurality of the votes of our unitholders at a special meeting or an annual meeting;
•: the successor general partner elected will be admitted to the Partnership as the general partner effective immediately prior to the withdrawal of the predecessor general partner;
•: the successor general partner elected will automatically become the successor general partner or managing member of our subsidiaries; and
•: if the successor general partner is elected, we will not be dissolved and the successor general partner will continue the business of us.

Transfer of General Partner Interests

The General Partner interests cannot be transferred to any person for any reason. So long as we are a limited partnership, 100% of the General Partner interests will be owned by the Partnership or one or more of its wholly owned subsidiaries.

Change of Management Provisions

The Amended and Restated Partnership Agreement contains specific provisions that are intended to discourage a person or group from attempting to remove MarkWest Energy GP, L.L.C. as the general partner or otherwise change management. If any person or group other than the General Partner and its affiliates acquires beneficial ownership of 20% or more of any class of units, that person

Table of Contents

or group loses voting rights on all of its units. This loss of voting rights does not apply to any person or group who acquires the units with the prior approval of the board of the General Partner.

Meetings; Voting

Except as described below regarding a person or group owning 20% or more of any class of units then outstanding and except for Class A units and Class B units as described above, unitholders or assignees who are record holders of our common units on the record date will be entitled to notice of, and to vote at, meetings of our limited partners and to act upon matters for which approvals may be solicited. Our common units that are owned by an assignee who is a record holder, but who has not yet been admitted as a limited partner, will be voted by the General Partner at the written direction of the record holder. Absent direction of this kind, our common units will not be voted.

Any action that is required or permitted to be taken by unitholders may be taken either at a meeting of our unitholders or without a meeting if consents in writing describing the action so taken are signed by holders of the number of units necessary to authorize or take that action at a meeting. Meetings of our unitholders may be called by the General Partner or by our unitholders owning at least 20% of our outstanding common units. Our unitholders may vote either in person or by proxy at meetings. The holders of a majority of the outstanding common units, represented in person or by proxy, will constitute a quorum unless any action by our unitholders requires approval by holders of a greater percentage of our common units, in which case the quorum will be the greater percentage.

However, if at any time any person or group, other than the General Partner and its affiliates, or a direct or subsequently approved transferee of the General Partner or its affiliates, or any person or group who acquires the units with the prior approval of the General Partner, acquires, in the aggregate, beneficial ownership of 20% or more of any class of units then outstanding, that person or group will lose voting rights on all of its units and the units may not be voted on any matter and will not be considered to be outstanding when sending notices of a meeting of unitholders, calculating required votes, determining the presence of a quorum or for other similar purposes. Our common units held in nominee or street name account will be voted by the broker or other nominee in accordance with the instruction of the beneficial owner unless the arrangement between the beneficial owner and his nominee provides otherwise.

Any notice, demand, request, report or proxy material required or permitted to be given or made to record holders of our common units under the Amended and Restated Partnership Agreement will be delivered to the record holder by us or by the transfer agent.

The annual meeting of the limited partners holding our common units will be held in June each year or on such other date and time as may be fixed from time to time by the General Partner. Notice of the annual meeting will be given between ten days and 60 days prior to the meeting date. The limited partners holding our common units will vote together as a single class. The limited partners entitled to vote will elect by plurality of the votes cast at such meeting the directors of the General Partner. Each of our common units is entitled to one vote for each of our common units that is registered in the name of the limited partner on the record date for the meeting. The General Partner, the Partnership or their affiliates cannot vote their units that they are otherwise entitled to vote, and those units are not considered to be outstanding for purposes of determining a quorum.

Board of Directors

The number of directors of the General Partner board will be between seven and eleven, but the number of the directors may be changed by the majority of the directors of the General Partner board so long as the decrease in number does not shorten the term of any incumbent director. Unless otherwise previously elected at a special meeting, at each annual meeting of the limited partners, the directors will be elected to hold office until the next annual meeting. Each director will hold office for

Table of Contents

the term for which such director is elected or until such director's earlier death, resignation or removal. Any vacancies may be filled, until the next annual meeting, by a majority of the remaining directors then in office. A director may be removed only for cause and only upon a vote of the majority of the remaining directors then in office. If the SEC promulgates a rule that provides for nominations by the stockholders or the unitholders of publicly traded companies of persons for election to the board of directors, the Partnership and the General Partner will adopt such rule as applied to a corporation without regard to any exemptions provided to limited partnerships.

Status as Limited Partner or Assignee

Except as described above under "—Limited Liability," our common units will be fully paid, and our unitholders will not be required to make additional contributions.

An assignee of one of our common units, after executing and delivering a transfer application, but pending its admission as a substituted limited partner, is entitled to an interest equivalent to that of a limited partner for the right to share in allocations and distributions from us, including liquidating distributions. The General Partner will vote and exercise other powers attributable to our common units owned by an assignee that has not become a substitute limited partner at the written direction of the assignee. Please read "—Meetings; Voting" above. Transferees that do not execute and deliver a transfer application will not be treated as assignees nor as record holders of our common units, and will not receive cash distributions, federal income tax allocations or reports furnished to holders of our common units. Please read "Description of the Common Units—Transfer of Common Units."

Non-citizen Assignees; Redemption

If we are or become subject to federal, state or local laws or regulations that, in the reasonable determination of the General Partner, create a substantial risk of cancellation or forfeiture of any property that we have an interest in because of the nationality, citizenship or other related status of any limited partner or assignee, we may redeem the units held by the limited partner or assignee at their current market price. In order to avoid any cancellation or forfeiture, the General Partner may require each limited partner or assignee to furnish information about his nationality, citizenship or related status. If a limited partner or assignee fails to furnish information about his nationality, citizenship or other related status within 30 days after a request for the information or the General Partner determines, with the advice of counsel, after receipt of the information that the limited partner or assignee is not an eligible citizen, the limited partner or assignee may be treated as a non-citizen assignee and the General Partner will be substituted for such non-citizen assignee as the limited partner in respect of his limited partner interests. In addition to other limitations on the rights of an assignee that is not a substituted limited partner, a non-citizen assignee does not have the right to direct the voting of his units and may not receive distributions in kind upon our liquidation.

Indemnification

Under the Amended and Restated Partnership Agreement, in most circumstances, we will indemnify the following persons, to the fullest extent permitted by law, from and against all losses, claims, damages or similar events:

•: the General Partner;
•: any person who is or was an affiliate of the General Partner or any departing general partner;
•: any person who is or was a member, partner, officer, director, employee, agent or trustee of us, Opco, any of their subsidiaries, the General Partner or any affiliate of us, Opco or any of their subsidiaries; or

Table of Contents

•: any person who is or was serving at the request of a general partner or any departing general partner or any affiliate of a general partner or any departing general partner as an officer, director, employee, member, partner, agent or trustee of another person.

Any indemnification under these provisions will only be out of our assets. The General Partner and its affiliates will not be personally liable for, or have any obligation to contribute or loan funds or assets to us to enable us to effectuate indemnification. We may purchase insurance against liabilities asserted against and expenses incurred by persons in connection with our activities, regardless of whether we would have the power to indemnify the person against liabilities under the Amended and Restated Partnership Agreement.

Books and Reports

The General Partner is required to keep appropriate books and records of our business at our principal offices. The books will be maintained for both tax and financial reporting purposes on an accrual basis. For tax and financial reporting purposes, our fiscal year is the calendar year.

We will furnish or make available to record holders of common units, within 120 days after the close of each fiscal year, an annual report containing audited financial statements and a report on those financial statements by our independent public accountants. Except for our fourth quarter, we will also furnish or make available summary financial information within 90 days after the close of each quarter.

We will furnish each record holder of a unit with information reasonably required for tax reporting purposes within 90 days after the close of each calendar year. This information is expected to be furnished in summary form so that some complex calculations normally required of partners can be avoided. Our ability to furnish this summary information to unitholders will depend on the cooperation of unitholders in supplying us with specific information. Every unitholder will receive information to assist him in determining his federal and state tax liability and filing his federal and state income tax returns, regardless of whether he supplies us with information.

Right to Inspect the Partnership's Books and Records

The Amended and Restated Partnership Agreement provides that a limited partner can, for a purpose reasonably related to his interest as a limited partner, upon reasonable written demand and at his own expense, have furnished to him:

•: a current list of the name and last known address of each limited partner;
•: a copy of our tax returns;
•: information as to the amount of cash, and a description and statement of the agreed value of any other property or services, contributed or to be contributed by each limited partner and the date on which each became a limited partner;
•: copies of the Amended and Restated Partnership Agreement, our certificate of limited partnership, related amendments and powers of attorney under which they have been executed;
•: information regarding the status of our business and financial condition; and
•: any other information regarding our affairs as is just and reasonable.

The General Partner may, and intends to, keep confidential from the limited partners trade secrets or other information the disclosure of which the general partner believes in good faith is not in our or our subsidiaries' best interests, could damage us or our subsidiaries or which we or our subsidiaries are required by law or by agreements with third parties to keep confidential.

Table of Contents

MATERIAL TAX CONSEQUENCES

This section summarizes the material U.S. federal income tax consequences that may be relevant to prospective unitholders and is based upon current provisions of the Code, existing and proposed U.S. Treasury regulations thereunder (the "Treasury Regulations"), and current administrative rulings and court decisions, all of which are subject to change. Changes in these authorities may cause the federal income tax consequences to a prospective unitholder to vary substantially from those described below. Unless the context otherwise requires, references in this section to "we" or "us" are references to MarkWest Energy Partners, L.P. and our subsidiaries.

Legal conclusions contained in this section, unless otherwise noted, are the opinion of Vinson & Elkins L.L.P. and are based on the accuracy of representations made by us to them for this purpose. However, this section does not address all federal income tax matters that affect us or our unitholders and does not describe the application of the alternative minimum tax that may be applicable to certain unitholders. Furthermore, this section focuses on unitholders who are individual citizens or residents of the United States (for federal income tax purposes), who have the U.S. dollar as their functional currency and who hold units as capital assets (generally, property that is held for investment). This section has limited applicability to corporations, partnerships (including entities treated as partnerships for U.S. federal income tax purposes), estates, trusts, non-resident aliens or other unitholders subject to specialized tax treatment, such as tax-exempt institutions, non-U.S. persons, individual retirement accounts ("IRAs"), employee benefit plans, real estate investment trusts or mutual funds.Accordingly, we encourage each unitholder to consult the unitholder's own tax advisor in analyzing the federal, state, local and non-U.S. tax consequences particular to that unitholder resulting from ownership or disposition of units and potential changes in applicable tax laws.

We are relying on opinions and advice of Vinson & Elkins L.L.P. with respect to the matters described herein. An opinion of counsel represents only that counsel's best legal judgment and does not bind the Internal Revenue Service (the "IRS") or a court. Accordingly, the opinions and statements made herein may not be sustained by a court if contested by the IRS. Any such contest of the matters described herein may materially and adversely impact the market for our common units and the prices at which our common units trade. In addition, our costs of any contest with the IRS will be borne indirectly by our unitholders and our general partner because the costs will reduce our cash available for distribution. Furthermore, the tax consequences of an investment in us may be significantly modified by future legislative or administrative changes or court decisions, which may be retroactively applied.

For the reasons described below, Vinson & Elkins L.L.P. has not rendered an opinion with respect to the following federal income tax issues: (1) the treatment of a unitholder whose units are the subject of a securities loan (e.g., a loan to a short seller to cover a short sale of units) (please read "—Tax Consequences of Unit Ownership—Treatment of Securities Loans"); (2) whether our monthly convention for allocating taxable income and losses is permitted by existing Treasury Regulations (please read "—Disposition of Units—Allocations Between Transferors and Transferees"); and (3) whether our method for taking into account Section 743 adjustments is sustainable in certain cases (please read "—Tax Consequences of Unit Ownership—Section 754 Election" and "—Uniformity of Units").

Taxation of the Partnership

Partnership Status

We expect to be treated as a partnership for U.S. federal income tax purposes and, therefore, generally will not be liable for entity-level federal income taxes. Instead, as described below, each of our unitholders will take into account its respective share of our items of income, gain, loss and deduction in computing its federal income tax liability as if the unitholder had earned such income

Table of Contents

directly, even if we make no cash distributions to the unitholder. Distributions we make to a unitholder generally will not give rise to income or gain taxable to such unitholder unless the amount of cash distributed exceeds the unitholder's adjusted tax basis in its units.

Section 7704 of the Code generally provides that publicly traded partnerships will be treated as corporations for federal income tax purposes. However, if 90% or more of a partnership's gross income for every taxable year it is publicly traded consists of "qualifying income," the partnership may continue to be treated as a partnership for federal income tax purposes (the "Qualifying Income Exception"). Qualifying income includes income and gains derived from the exploration and production, refining, transportation, storage, processing and marketing of certain natural resources, including crude oil, natural gas, and products thereof. Other types of qualifying income include interest (other than from a financial business), dividends, gains from the sale of real property and gains from the sale or other disposition of capital assets held for the production of qualifying income. We estimate that less than 2% of our current gross income is not qualifying income; however, this estimate could change from time to time.

Based upon factual representations made by us and our general partner regarding the composition of our income and the other representations set forth below, Vinson & Elkins L.L.P. is of the opinion that we will be treated as a partnership for federal income tax purposes and each of our operating subsidiaries will be disregarded as an entity separate from us for federal income tax purposes. In rendering its opinion, Vinson & Elkins L.L.P. has relied on factual representations made by us and our general partner. The representations made by us and our general partner upon which Vinson & Elkins L.L.P. has relied include, without limitation:

(a) Neither we nor any of our operating subsidiaries has elected to be treated as a corporation for federal income tax purposes;

(b) For each taxable year, since and including the year of our initial public offering, more than 90% of our gross income has been and will be income of a character that Vinson & Elkins L.L.P. has opined is "qualifying income" within the meaning of Section 7704(d) of the Code; and

(c) Each hedging transaction that we treat as resulting in qualifying income has been and will be appropriately identified as a hedging transaction pursuant to applicable Treasury Regulations, and has been and will be associated with oil, natural gas, or products thereof that are held or to be held by us in activities that Vinson & Elkins, L.L.P. has opined or will opine result in qualifying income.

We believe that these representations are true and will be true in the future.

If we fail to meet the Qualifying Income Exception, other than a failure that is determined by the IRS to be inadvertent and that is cured within a reasonable time after discovery (in which case the IRS may also require us to make adjustments with respect to our unitholders or pay other amounts), we will be treated as transferring all of our assets, subject to liabilities, to a newly formed corporation, on the first day of the year in which we fail to meet the Qualifying Income Exception, in return for stock in that corporation and then as distributing that stock to our unitholders in liquidation. This deemed contribution and liquidation should not result in the recognition of taxable income by our unitholders or us so long as our liabilities do not exceed the tax basis of our assets. Thereafter, we would be treated as an association taxable as a corporation for federal income tax purposes.

The present federal income tax treatment of publicly traded partnerships, including us, or an investment in our common units may be modified by administrative, legislative, or judicial interpretation at any time. For example, from time to time, members of the U.S. Congress propose and consider substantive changes to the existing federal income tax laws that affect publicly traded partnerships. Currently, one such legislative proposal would eliminate the qualifying income exception

Table of Contents

upon which we rely for our treatment as a partnership for U.S. federal income tax purposes. We are unable to predict whether any such changes will ultimately be enacted. However, it is possible that a change in law could affect us and may be applied retroactively. Any such changes could negatively impact the value of an investment in our common units.

If for any reason we are taxable as a corporation in any taxable year, our items of income, gain, loss and deduction would be taken into account by us in determining the amount of our liability for federal income tax, rather than being passed through to our unitholders. Our taxation as a corporation would materially reduce the cash available for distribution to unitholders and thus would likely substantially reduce the value of our common units. Any distribution made to a unitholder at a time we are treated as a corporation would be (i) a taxable dividend to the extent of our current or accumulated earnings and profits, then (ii) a nontaxable return of capital to the extent of the unitholder's tax basis in its units, and thereafter (iii) taxable capital gain.

The remainder of this discussion is based on the opinion of Vinson & Elkins L.L.P. that we will be treated as a partnership for federal income tax purposes.

Tax Consequences of Unit Ownership

Limited Partner Status

Unitholders who are admitted as limited partners of the Partnership will be treated as partners of the Partnership for federal income tax purposes. Also:

(a): assignees who have executed and delivered transfer applications, and are awaiting admission as limited partners, and
(b): unitholders whose common units are held in street name or by a nominee and who have the right to direct the nominee in the exercise of all substantive rights attendant to the ownership of their common units,

will be treated as partners of the Partnership for federal income tax purposes.

As there is no direct or indirect controlling authority addressing the federal tax treatment of assignees of common units who are entitled to execute and deliver transfer applications and thereby become entitled to direct the exercise of attendant rights, but who fail to execute and deliver transfer applications, the opinion of Vinson & Elkins L.L.P. does not extend to these persons. Furthermore, a purchaser or other transferee of common units who does not execute and deliver a transfer application may not receive some federal income tax information or reports furnished to record holders of common units unless the common units are held in a nominee or street name account and the nominee or broker has executed and delivered a transfer application for those common units.

For a discussion related to the risks of losing partner status as a result of securities loans, please read "—Treatment of Securities Loans." Unitholders who are not treated as partners of the partnership as described above are urged to consult their own tax advisors with respect to the tax consequences applicable to them under the circumstances.

Flow-Through of Taxable Income

Subject to the discussion below under "—Entity-Level Collections of Unitholder Taxes" with respect to payments we may be required to make on behalf of our unitholders, we will not pay any federal income tax. Rather, each unitholder will be required to report on its federal income tax return each year its share of our income, gains, losses and deductions for our taxable year or years ending with or within its taxable year. Consequently, we may allocate income to a unitholder even if that unitholder has not received a cash distribution.

Table of Contents

Basis of Units

A unitholder's tax basis in its units initially will be the amount paid for those units plus the unitholder's share of our liabilities. That basis generally will be (i) increased by the unitholder's share of our income and any increases in such unitholder's share of our liabilities, and (ii) decreased, but not below zero, by the amount of all distributions, the unitholder's share of our losses, and any decreases in its share of our liabilities.

Treatment of Distributions

Distributions made by us to a unitholder generally will not be taxable to the unitholder, unless such distributions exceed the unitholder's tax basis in its units, in which case the unitholder generally will recognize gain taxable in the manner described below under "—Disposition of Units."

Any reduction in a unitholder's share of our liabilities will be treated as a distribution by us of cash to that unitholder. A decrease in a unitholder's percentage interest in us because of our issuance of additional units may decrease the unitholder's share of our liabilities. For purposes of the foregoing, a unitholder's share of our nonrecourse liabilities (liabilities for which no partner bears the economic risk of loss) generally will be based upon that unitholder's share of the unrealized appreciation (or depreciation) in our assets, to the extent thereof, with any excess liabilities allocated based on the unitholder's share of our profits. Please read "—Disposition of Units."

A non-pro rata distribution of money or property (including a deemed distribution as a result of the reduction in a unitholder's share of our liabilities as described above) may cause a unitholder to recognize ordinary income, if the distribution reduces the unitholder's share of our "unrealized receivables," including depreciation recapture and substantially appreciated "inventory items," both as defined in Section 751 of the Code ("Section 751 Assets"). To the extent of such reduction, the unitholder would be deemed to receive its proportionate share of the Section 751 Assets and exchange such assets with us in return for a portion of the non-pro rata distribution. This deemed exchange generally will result in the unitholder's recognition of ordinary income in an amount equal to the excess of (1) the non-pro rata portion of that distribution over (2) the unitholder's tax basis (generally zero) in the Section 751 Assets deemed to be relinquished in the exchange.

Limitations on Deductibility of Losses

A unitholder may not be entitled to deduct the full amount of loss we allocate to it because its share of our losses will be limited to the lesser of (i) the unitholder's tax basis in its units, and (ii) in the case of a unitholder that is an individual, estate, trust or certain types of closely-held corporations, the amount for which the unitholder is considered to be "at risk" with respect to our activities. In general, a unitholder will be at risk to the extent of its tax basis in its units, reduced by (1) any portion of that basis attributable to the unitholder's share of our liabilities, (2) any portion of that basis representing amounts otherwise protected against loss because of a guarantee, stop loss agreement or similar arrangement and (3) any amount of money the unitholder borrows to acquire or hold its units, if the lender of those borrowed funds owns an interest in us, is related to another unitholder or can look only to the units for repayment. A unitholder subject to the at risk limitation must recapture losses deducted in previous years to the extent that distributions (including distributions deemed to result from a reduction in a unitholder's share of nonrecourse liabilities) cause the unitholder's at risk amount to be less than zero at the end of any taxable year.

Losses disallowed to a unitholder or recaptured as a result of the basis or at risk limitations will carry forward and will be allowable as a deduction in a later year to the extent that the unitholder's tax basis or at risk amount, whichever is the limiting factor, is subsequently increased. Upon a taxable disposition of units, any gain recognized by a unitholder can be offset by losses that were previously suspended by the at risk limitation but not losses suspended by the basis limitation. Any loss previously

Table of Contents

suspended by the at risk limitation in excess of that gain can no longer be used, and will not be available to offset a unitholder's salary or active business income.

In addition to the basis and at risk limitations, a passive activity loss limitation generally limits the deductibility of losses incurred by individuals, estates, trusts, some closely held corporations and personal service corporations from "passive activities" (generally, trade or business activities in which the taxpayer does not materially participate). The passive loss limitations are applied separately with respect to each publicly-traded partnership. Consequently, any passive losses we generate will be available to offset only passive income generated by us Passive losses that exceed a unitholder's share of passive income we generate may be deducted in full when the unitholder disposes of all of its units in a fully taxable transaction with an unrelated party. The passive loss rules generally are applied after other applicable limitations on deductions, including the at risk and basis limitations.

Limitations on Interest Deductions

The deductibility of a non-corporate taxpayer's "investment interest expense" generally is limited to the amount of that taxpayer's "net investment income." Investment interest expense includes:

•: interest on indebtedness properly allocable to property held for investment;
•: interest expense allocated against portfolio income; and
•: the portion of interest expense incurred to purchase or carry an interest in a passive activity to the extent allocable against portfolio income.

The computation of a unitholder's investment interest expense will take into account interest on any margin account borrowing or other loan incurred to purchase or carry a unit. Net investment income includes gross income from property held for investment and amounts treated as portfolio income under the passive loss rules, less deductible expenses, other than interest, directly connected with the production of investment income. Net investment income generally does not include qualified dividend income (if applicable) or gains attributable to the disposition of property held for investment. A unitholder's share of a publicly traded partnership's portfolio income and, according to the IRS, net passive income will be treated as investment income for purposes of the investment interest expense limitation.

Entity-Level Collections of Unitholder Taxes

If we are required or elect under applicable law to pay any federal, state, local or non-U.S. tax on behalf of any current or former unitholder or our general partner, we are authorized to treat the payment as a distribution of cash to the relevant unitholder or general partner. Where the tax is payable on behalf of all the unitholders or we cannot determine the specific unitholder on whose behalf the tax is payable, we are authorized to treat the payment as a distribution to all current unitholders. Payments by us as described above could give rise to an overpayment of tax on behalf of a unitholder, in which event the unitholder may be entitled to claim a refund of the overpayment amount. Unitholders are urged to consult their tax advisors to determine the consequences to them of any tax payment we make on their behalf.

Allocation of Income, Gain, Loss and Deduction

In general, our items of income, gain, loss and deduction will be allocated amongst our unitholders in accordance with their percentage interests in us. However, in addition to allocations with respect to any Book-Tax Disparities, as described below, our Amended and Restated Partnership Agreement provides for special allocations to achieve economic uniformity between the Class B Units and the common units and allocate a portion of the Partnership's income away from the Class B Units to the extent that the common units receive distributions to which the Class B Units are not entitled.

Table of Contents

Specified items of our income, gain, loss and deduction will be allocated under Section 704(c) of the Code (or the principles of Section 704(c) of the Code) to account for any difference between the tax basis and fair market value of our assets at the time such assets are contributed to us and at the time of any subsequent offering of our common units (a "Book-Tax Disparity"). As a result, the federal income tax burden associated with any Book-Tax Disparity immediately prior to an offering generally will be borne by our partners holding interests in us prior to such offering. In addition, items of recapture income will be specially allocated to the extent possible to the unitholder who was allocated the deduction giving rise to that recapture income in order to minimize the recognition of ordinary income by other unitholders.

An allocation of items of our income, gain, loss or deduction, other than an allocation required by the Code to eliminate a Book-Tax Disparity, will generally be given effect for federal income tax purposes in determining a partner's share of an item of income, gain, loss or deduction only if the allocation has "substantial economic effect." In any other case, a partner's share of an item will be determined on the basis of the partner's interest in us, which will be determined by taking into account all the facts and circumstances, including (i) his relative contributions to us, (ii) the interests of all the partners in profits and losses, (iii) the interest of all the partners in cash flow and (iv) the rights of all the partners to distributions of capital upon liquidation. Vinson & Elkins L.L.P. is of the opinion that, with the exception of the issues described in "—Section 754 Election" and "—Disposition of Units—Allocations Between Transferors and Transferees," allocations under our Amended and Restated Partnership Agreement will be given effect for federal income tax purposes in determining a partner's share of an item of income, gain, loss or deduction.

Treatment of Securities Loans

A unitholder whose units are loaned (for example, a loan to a "short seller" to cover a short sale of units) may be treated as having disposed of those units. If so, such unitholder would no longer be treated for tax purposes as a partner with respect to those units during the period of the loan and may recognize gain or loss from the disposition. As a result, during this period (i) any of our income, gain, loss or deduction allocated to those units would not be reportable by the lending unitholder and (ii) any cash distributions received by the unitholder as to those units may be treated as ordinary taxable income.

Due to a lack of controlling authority, Vinson & Elkins L.L.P. has not rendered an opinion regarding the tax treatment of a unitholder that enters into a securities loan with respect to its units. Unitholders desiring to assure their status as partners and avoid the risk of income recognition from a loan of their units are urged to modify any applicable brokerage account agreements to prohibit their brokers from borrowing and lending their units. The IRS has announced that it is studying issues relating to the tax treatment of short sales of partnership interests. Please read "—Disposition of Units—Recognition of Gain or Loss."

Tax Rates

Under current law, the highest marginal federal income tax rates for individuals applicable to ordinary income and long-term capital gains (generally, gains from the sale or exchange of certain investment assets held for more than one year) are 35% and 15%, respectively. However, absent new legislation extending the current rates, beginning January 1, 2013, the highest marginal federal income tax rate applicable to ordinary income and long-term capital gains of individuals will increase to 39.6% and 20%, respectively. These rates are subject to change by new legislation at any time.

A 3.8% Medicare tax on certain net investment income earned by individuals, estates, and trusts will apply for taxable years beginning after December 31, 2012. For these purposes, net investment income generally includes a unitholder's allocable share of our income and gain realized by a

Table of Contents

unitholder from a sale of units. In the case of an individual, the tax will be imposed on the lesser of (i) the unitholder's net investment income from all investments, or (ii) the amount by which the unitholder's modified adjusted gross income exceeds $250,000 (if the unitholder is married and filing jointly or a surviving spouse), $125,000 (if married filing separately) or $200,000 (if the unitholder is unmarried or in any other case). In the case of an estate or trust, the tax will be imposed on the lesser of (i) undistributed net investment income, or (ii) the excess adjusted gross income over the dollar amount at which the highest income tax bracket applicable to an estate or trust begins.

Section 754 Election

We have made the election permitted by Section 754 of the Code that permits us to adjust the tax bases in our assets as to specific purchasers of our common units under Section 743(b) of the Code. The Section 743(b) adjustment separately applies to each purchaser of units based upon the values and bases of our assets at the time of the relevant purchase, and the adjustment will reflect the purchase price paid. The Section 743(b) adjustment does not apply to a person who purchases units directly from us.

Under our Amended and Restated Partnership Agreement, we are authorized to take a position to preserve the uniformity of units even if that position is not consistent with applicable Treasury Regulations. A literal application of Treasury Regulations governing a 743(b) adjustment attributable to properties depreciable under Section 167 of the Code may give rise to differences in the taxation of unitholders purchasing units from us and unitholders purchasing from other unitholders. If we have any such properties, we intend to adopt methods employed by other publicly traded partnerships to preserve the uniformity of units, even if inconsistent with existing Treasury Regulations, and Vinson & Elkins, L.L.P. has not opined on the validity of this approach. Please read "—Uniformity of Units."

The IRS may challenge the positions we adopt with respect to depreciating or amortizing the Section 743(b) adjustment we take to preserve the uniformity of units due to lack of controlling authority. Because a unitholder's tax basis for its units is reduced by its share of our items of deduction or loss, any position we take that understates deductions will overstate a unitholder's basis in its units, and may cause the unitholder to understate gain or overstate loss on any sale of such units. Please read "—Disposition of Units—Recognition of Gain or Loss." If a challenge to such treatment were sustained, the gain from the sale of units may be increased without the benefit of additional deductions.

The calculations involved in the Section 754 election are complex and will be made on the basis of assumptions as to the value of our assets and other matters. The IRS could seek to reallocate some or all of any Section 743(b) adjustment we allocated to our assets subject to depreciation to goodwill or nondepreciable assets. Goodwill, as an intangible asset, is generally nonamortizable or amortizable over a longer period of time or under a less accelerated method than our tangible assets. We cannot assure any unitholder that the determinations we make will not be successfully challenged by the IRS or that the resulting deductions will not be reduced or disallowed altogether. Should the IRS require a different tax basis adjustment to be made, and should, in our opinion, the expense of compliance exceed the benefit of the election, we may seek permission from the IRS to revoke our Section 754 election. If permission is granted, a subsequent purchaser of units may be allocated more income than it would have been allocated had the election not been revoked.

Tax Treatment of Operations

Accounting Method and Taxable Year

We will use the year ending December 31 as our taxable year and the accrual method of accounting for federal income tax purposes. Each unitholder will be required to include in income its share of our income, gain, loss and deduction for each taxable year ending within or with its taxable year. In addition, a unitholder who has a taxable year ending on a date other than December 31 and

Table of Contents

who disposes of all of its units following the close of our taxable year but before the close of its taxable year must include its share of our income, gain, loss and deduction in income for its taxable year, with the result that it will be required to include in income for its taxable year its share of more than one year of our income, gain, loss and deduction. Please read "—Disposition of Units—Allocations Between Transferors and Transferees."

Tax Basis, Depreciation and Amortization

The tax bases of our assets will be used for purposes of computing depreciation and cost recovery deductions and, ultimately, gain or loss on the disposition of these assets. If we dispose of depreciable property by sale, foreclosure or otherwise, all or a portion of any gain, determined by reference to the amount of depreciation deductions previously taken, may be subject to the recapture rules and taxed as ordinary income rather than capital gain. Similarly, a unitholder who has taken cost recovery or depreciation deductions with respect to property we own will likely be required to recapture some or all of those deductions as ordinary income upon a sale of its interest in us. Please read "—Tax Consequences of Unit Ownership—Allocation of Income, Gain, Loss and Deduction" and "—Disposition of Units—Recognition of Gain or Loss."

The costs we incur in offering and selling our common units (called "syndication expenses") must be capitalized and cannot be deducted currently, ratably or upon our termination. While there are uncertainties regarding the classification of costs as organization expenses, which may be amortized by us, and as syndication expenses, which may not be amortized by us, the underwriting discounts and commissions we incur will be treated as syndication expenses.

Valuation and Tax Bases of Our Properties

The federal income tax consequences of the ownership and disposition of units will depend in part on our estimates of the relative fair market values and the initial tax bases of our assets. Although we may from time to time consult with professional appraisers regarding valuation matters, we will make many of the relative fair market value estimates ourselves. These estimates and determinations of tax basis are subject to challenge and will not be binding on the IRS or the courts. If the estimates of fair market value or basis are later found to be incorrect, the character and amount of items of income, gain, loss or deduction previously reported by unitholders could change, and unitholders could be required to adjust their tax liability for prior years and incur interest and penalties with respect to those adjustments.

Disposition of Units

Recognition of Gain or Loss

A unitholder will be required to recognize gain or loss on a sale of units equal to the difference between the unitholder's amount realized and tax basis in the units sold. A unitholder's amount realized generally will equal the sum of the cash or the fair market value of other property it receives plus its share of our liabilities with respect to such units. Because the amount realized includes a unitholder's share of our liabilities, the gain recognized on the sale of units could result in a tax liability in excess of any cash received from the sale.

Except as noted below, gain or loss recognized by a unitholder on the sale or exchange of a unit held for more than one year generally will be taxable as long-term capital gain or loss. However, gain or loss recognized on the disposition of units will be separately computed and taxed as ordinary income or loss under Section 751 of the Code to the extent attributable to Section 751 Assets, such as depreciation recapture. Ordinary income attributable to Section 751 Assets may exceed net taxable gain realized on the sale of a unit and may be recognized even if there is a net taxable loss realized on the sale of a unit. Thus, a unitholder may recognize both ordinary income and capital gain or loss upon a

Table of Contents

sale of units. Net capital loss may offset capital gains and, in the case of individuals, up to $3,000 of ordinary income per year.

The IRS has ruled that a partner who acquires interests in a partnership in separate transactions must combine those interests and maintain a single adjusted tax basis for all those interests. Upon a sale or other disposition of less than all of those interests, a portion of that tax basis must be allocated to the interests sold using an "equitable apportionment" method, which generally means that the tax basis allocated to the interest sold equals an amount that bears the same relation to the partner's tax basis in its entire interest in the partnership as the value of the interest sold bears to the value of the partner's entire interest in the partnership.

Treasury Regulations under Section 1223 of the Code allow a selling unitholder who can identify units transferred with an ascertainable holding period to elect to use the actual holding period of the units transferred. Thus, according to the ruling discussed above, a unitholder will be unable to select high or low basis units to sell as would be the case with corporate stock, but, according to the Treasury Regulations, it may designate specific units sold for purposes of determining the holding period of units transferred. A unitholder electing to use the actual holding period of units transferred must consistently use that identification method for all subsequent sales or exchanges of our common units. A unitholder considering the purchase of additional units or a sale of units purchased in separate transactions is urged to consult its tax advisor as to the possible consequences of this ruling and application of the Treasury Regulations.

Specific provisions of the Code affect the taxation of some financial products and securities, including partnership interests, by treating a taxpayer as having sold an "appreciated" financial position, including a partnership interest with respect to which gain would be recognized if it were sold, assigned or terminated at its fair market value, in the event the taxpayer or a related person enters into:

•: a short sale;
•: an offsetting notional principal contract; or
•: a futures or forward contract with respect to the partnership interest or substantially identical property.

Moreover, if a taxpayer has previously entered into a short sale, an offsetting notional principal contract or a futures or forward contract with respect to the partnership interest, the taxpayer will be treated as having sold that position if the taxpayer or a related person then acquires the partnership interest or substantially identical property. The Secretary of the Treasury is authorized to issue regulations that treat a taxpayer that enters into transactions or positions that have substantially the same effect as the preceding transactions as having constructively sold the financial position.

Allocations Between Transferors and Transferees

In general, our taxable income or loss will be determined annually, will be prorated on a monthly basis and will be subsequently apportioned among the unitholders in proportion to the number of units owned by each of them as of the opening of the applicable exchange on the first business day of the month (the "Allocation Date"). However, gain or loss realized on a sale or other disposition of our assets or, in the discretion of the general partner, any other extraordinary item of income, gain, loss or deduction will be allocated among the unitholders on the Allocation Date in the month in which such income, gain, loss or deduction is recognized. As a result, a unitholder transferring units may be allocated income, gain, loss and deduction realized after the date of transfer.

Although simplifying conventions are contemplated by the Code and most publicly traded partnerships use similar simplifying conventions, the use of this method may not be permitted under existing Treasury Regulations. Recently, however, the Department of the Treasury and the IRS issued

Table of Contents

proposed Treasury Regulations that provide a safe harbor pursuant to which a publicly traded partnership may use a similar monthly simplifying convention to allocate tax items among transferor and transferee unitholders, although such tax items must be prorated on a daily basis. Nonetheless, the proposed regulations do not specifically authorize the use of the proration method we have adopted. Accordingly, Vinson & Elkins L.L.P. is unable to opine on the validity of this method of allocating income and deductions between transferee and transferor unitholders. If this method is not allowed under the final Treasury Regulations, or only applies to transfers of less than all of the unitholder's interest, our taxable income or losses could be reallocated among our unitholders. We are authorized to revise our method of allocation between transferee and transferor unitholders, as well as among unitholders whose interests vary during a taxable year, to conform to a method permitted under future Treasury Regulations.

A unitholder who disposes of units prior to the record date set for a cash distribution for that quarter will be allocated items of our income, gain, loss and deduction attributable to the month of disposition but will not be entitled to receive a cash distribution for that period.

Notification Requirements

A unitholder who sells or purchases any of its units is generally required to notify us in writing of that transaction within 30 days after the transaction (or, if earlier, January 15 of the year following the transaction in the case of a seller). Upon receiving such notifications, we are required to notify the IRS of that transaction and to furnish specified information to the transferor and transferee. Failure to notify us of a transfer of units may, in some cases, lead to the imposition of penalties. However, these reporting requirements do not apply to a sale by an individual who is a citizen of the United States and who effects the sale through a broker who will satisfy such requirements.

Constructive Termination

We will be considered to have "constructively" terminated as a partnership for federal income tax purposes upon the sale or exchange of 50% or more of the total interests in our capital and profits within a twelve-month period. For such purposes, multiple sales of the same unit are counted only once. A constructive termination results in the closing of our taxable year for all unitholders. In the case of a unitholder reporting on a taxable year other than the calendar year, the closing of our taxable year may result in more than twelve months of our taxable income or loss being includable in such unitholder's taxable income for the year of termination.

A constructive termination occurring on a date other than December 31 generally would require that we file two tax returns for one fiscal year, and the cost of the preparation of these returns will be borne by all unitholders. However, pursuant to an IRS relief procedure, the IRS may allow a constructively terminated partnership to provide a single Schedule K-1 for the calendar year in which a termination occurs. Following a constructive termination, we would be required to make new tax elections, including a new election under Section 754 of the Code, and the termination would result in a deferral of our deductions for depreciation. A termination could also result in penalties if we were unable to determine that the termination had occurred. Moreover, a termination may either accelerate the application of, or subject us to, any tax legislation enacted before the termination that would not otherwise have been applied to us as a continuing partnership, as opposed to a terminating partnership.

Uniformity of Units

Because we cannot match transferors and transferees of units and for other reasons, we must maintain uniformity of the economic and tax characteristics of the units to a purchaser of these units. In the absence of uniformity, we may be unable to completely comply with a number of federal income

Table of Contents

tax requirements. Any non-uniformity could have a negative impact on the value of the units. Please read "—Tax Consequences of Unit Ownership—Section 754 Election."

Our Amended and Restated Partnership Agreement permits our general partner to take positions in filing our tax returns that preserve the uniformity of our common units. These positions may include reducing the depreciation, amortization or loss deductions to which a unitholder would otherwise be entitled or reporting a slower amortization of Section 743(b) adjustments for some unitholders than that to which they would otherwise be entitled. Vinson & Elkins L.L.P. is unable to opine as to validity of such filing positions.

A unitholder's basis in units is reduced by its share of our deductions (whether or not such deductions were claimed on an individual income tax return) so that any position that we take that understates deductions will overstate the unitholder's basis in its units, and may cause the unitholder to understate gain or overstate loss on any sale of such units. Please read "—Disposition of Units—Recognition of Gain or Loss" above and "—Tax Consequences of Unit Ownership—Section 754 Election" above. The IRS may challenge one or more of any positions we take to preserve the uniformity of units. If such a challenge were sustained, the uniformity of units might be affected, and, under some circumstances, the gain from the sale of units might be increased without the benefit of additional deductions.

Tax-Exempt Organizations and Other Investors

Ownership of units by employee benefit plans, other tax-exempt organizations, non-resident aliens, non-U.S. corporations and other non-U.S. persons raises issues unique to those investors and, as described below, may have substantially adverse tax consequences to them. Prospective unitholders that are tax-exempt entities or non-U.S. persons should consult their tax advisors before investing in our common units.

Employee benefit plans and most other tax-exempt organizations, including IRAs and other retirement plans, are subject to federal income tax on unrelated business taxable income. Virtually all of our income will be unrelated business taxable income and will be taxable to a tax-exempt unitholder.

Non-resident aliens and non-U.S. corporations, trusts or estates that own units will be treated as engaged in business in the United States because of their ownership of our common units. Consequently, they will be required to file federal tax returns to report their share of our income, gain, loss or deduction and pay federal income tax at regular rates on their share of our net income or gain. Moreover, under rules applicable to publicly traded partnerships, distributions to non-U.S. unitholders are subject to withholding at the highest applicable effective tax rate. Each non-U.S. unitholder must obtain a taxpayer identification number from the IRS and submit that number to our transfer agent on a Form W-8BEN or applicable substitute form in order to obtain credit for these withholding taxes.

In addition, because a non-U.S. corporation that owns units will be treated as engaged in a United States trade or business, that corporation may be subject to the U.S. branch profits tax at a rate of 30%, in addition to regular federal income tax, on its share of our income and gain to the extent reflected in earnings and profits, and as adjusted for changes in the foreign corporation's "U.S. net equity." That tax may be reduced or eliminated by an income tax treaty between the United States and the country in which the foreign corporate unitholder is a "qualified resident." In addition, this type of unitholder is subject to special information reporting requirements under Section 6038C of the Code.

A non-U.S. unitholder who sells or otherwise disposes of a unit will be subject to federal income tax on gain realized from the sale or disposition of that unit to the extent the gain is effectively connected with a U.S. trade or business of the non-U.S. unitholder. Under a ruling published by the IRS interpreting the scope of "effectively connected income," part or all of a non-U.S. unitholder's gain may be treated as effectively connected with that unitholder's indirect U.S. trade or business

Table of Contents

constituted by its investment in us. Moreover, under the Foreign Investment in Real Property Tax Act, a non-U.S. unitholder generally will be subject to federal income tax upon the sale or disposition of a unit if (i) it owned (directly or constructively applying certain attribution rules) more than 5% of our common units at any time during the five-year period ending on the date of such disposition and (ii) 50% or more of the fair market value of all of our assets consisted of U.S. real property interests at any time during the shorter of the period during which such unitholder held the units or the 5-year period ending on the date of disposition. More than 50% of our assets may consist of U.S. real property interests. Therefore, non-U.S. unitholders may be subject to federal income tax on gain from the sale or disposition of their units.

Administrative Matters

Information Returns and Audit Procedures

We intend to furnish to each unitholder, within 90 days after the close of each taxable year, specific tax information, including a Schedule K-1, which describes its share of our income, gain, loss and deduction for our preceding taxable year. In preparing this information, which will not be reviewed by counsel, we will take various accounting and reporting positions, some of which have been mentioned earlier, to determine each unitholder's share of income, gain, loss and deduction. We cannot assure our unitholders that those positions will yield a result that conforms to all of the requirements of the Code, Treasury Regulations or administrative interpretations of the IRS.

The IRS may audit our federal income tax information returns. Neither we nor Vinson & Elkins L.L.P. can assure prospective unitholders that the IRS will not successfully challenge the positions we adopt, and such a challenge could adversely affect the value of the units. Adjustments resulting from an IRS audit may require each unitholder to adjust a prior year's tax liability, and may result in an audit of the unitholder's own return. Any audit of a unitholder's return could result in adjustments unrelated to our returns.

Publicly traded partnerships generally are treated as entities separate from their owners for purposes of federal income tax audits, judicial review of administrative adjustments by the IRS, and tax settlement proceedings. The tax treatment of partnership items of income, gain, loss and deduction are determined in a partnership proceeding rather than in separate proceedings of the partners. The Code requires that one partner be designated as the "Tax Matters Partner" for these purposes, and our Amended and Restated Partnership Agreement designates our general partner.

The Tax Matters Partner can extend the statute of limitations for assessment of tax deficiencies against unitholders for items in our returns. The Tax Matters Partner may bind a unitholder with less than a 1% profits interest in us to a settlement with the IRS unless that unitholder elects, by filing a statement with the IRS, not to give that authority to the Tax Matters Partner. The Tax Matters Partner may seek judicial review, by which all the unitholders are bound, of a final partnership administrative adjustment and, if the Tax Matters Partner fails to seek judicial review, judicial review may be sought by any unitholder having at least a 1% interest in profits or by any group of unitholders having in the aggregate at least a 5% interest in profits. However, only one action for judicial review may go forward, and each unitholder with an interest in the outcome may participate in that action.

A unitholder must file a statement with the IRS identifying the treatment of any item on its federal income tax return that is not consistent with the treatment of the item on our return. Intentional or negligent disregard of this consistency requirement may subject a unitholder to substantial penalties.

Table of Contents

Nominee Reporting

Persons who hold an interest in us as a nominee for another person are required to furnish to us:

(1) the name, address and taxpayer identification number of the beneficial owner and the nominee;

(2) a statement regarding whether the beneficial owner is:

(a) a non-U.S. person;

(b) a non-U.S. government, an international organization or any wholly owned agency or instrumentality of either of the foregoing; or

(3) the amount and description of units held, acquired or transferred for the beneficial owner; and

(4) specific information including the dates of acquisitions and transfers, means of acquisitions and transfers, and acquisition cost for purchases, as well as the amount of net proceeds from sales.

Brokers and financial institutions are required to furnish additional information, including whether they are U.S. persons and specific information on units they acquire, hold or transfer for their own account. A penalty of $100 per failure, up to a maximum of $1.5 million per calendar year, is imposed by the Code for failure to report that information to us. The nominee is required to supply the beneficial owner of the units with the information furnished to us.

Accuracy-Related Penalties

An additional tax equal to 20% of the amount of any portion of an underpayment of tax that is attributable to one or more specified causes, including negligence or disregard of rules or regulations, substantial understatements of income tax and substantial valuation misstatements, is imposed by the Code. No penalty will be imposed, however, for any portion of an underpayment if it is shown that there was a reasonable cause for the underpayment of that portion and that the taxpayer acted in good faith regarding the underpayment of that portion.

State, Local and Other Tax Considerations

In addition to federal income taxes, unitholders may be subject to other taxes, including state and local income taxes, unincorporated business taxes, and estate, inheritance or intangibles taxes that may be imposed by the various jurisdictions in which we conduct business or own property now or in the future or in which the unitholder is a resident. We currently conduct business or own property in several states, many of which impose an income tax on nonresident partners of partnerships doing business within the state. We may also own property or do business in other states in the future that impose income or similar taxes on nonresident persons owning an interest in us. Although an analysis of those various taxes is not presented here, each prospective unitholder should consider their potential impact on its investment in us. It is the responsibility of each unitholder to investigate the legal and tax consequences, under the laws of pertinent states and localities, of its investment in us. Vinson & Elkins L.L.P. has not rendered an opinion on the state, local, alternative minimum tax or non-U.S. tax consequences of an investment in us. We strongly recommend that each prospective unitholder consult, and depend on, its own tax counsel or other advisor with regard to those matters. It is the responsibility of each unitholder to file all tax returns that may be required of it.

Table of Contents

Tax Consequences of Ownership of Debt Securities

A description of the material federal income tax consequences of the acquisition, ownership and disposition of debt securities will be set forth on the prospectus supplement relating to the offering of debt securities.

Table of Contents

INVESTMENT IN MARKWEST ENERGY PARTNERS BY EMPLOYEE BENEFIT PLANS

An investment in our common units or debt securities by an employee benefit plan is subject to certain additional considerations because the investments of these plans are subject to the fiduciary responsibility and prohibited transaction provisions of the Employee Retirement Income Security Act of 1974, as amended ("ERISA"), and restrictions imposed by Section 4975 of the Code, and provisions under any federal, state, local, non-U.S. or other laws or regulations that are similar to such provisions of the Code or ERISA (collectively, "Similar Laws"). As used herein, the term "employee benefit plan" includes, but is not limited to, qualified pension, profit sharing and stock bonus plans, Keogh plans, simplified employee pension plans and tax deferred annuities or individual retirement accounts ("IRAs") or other arrangements established or maintained by an employer or employee organization, and entities whose underlying assets are considered to include "plan assets" of such plans, accounts and arrangements.

General Fiduciary Matters

ERISA and the Code impose certain duties on persons who are fiduciaries of an employee benefit plan that is subject to Title I of ERISA or Section 4975 of the Code (an "ERISA Plan") and prohibit certain transactions involving the assets of an ERISA Plan and its fiduciaries or other interested parties. Under ERISA and the Code, any person who exercises any discretionary authority or control over the administration of such an ERISA Plan or the management or disposition of the assets of such an ERISA Plan, or who renders investment advice for a fee or other compensation to such an ERISA Plan, is generally considered to be a fiduciary of the ERISA Plan. In considering an investment in our common units or debt securities, among other things, consideration should be given to:

(a): whether the investment is prudent under Section 404(a)(1)(B) of ERISA and any other applicable Similar Laws;
(b): whether in making the investment, that plan will satisfy the diversification requirements of Section 404(a)(1)(C) of ERISA and any other applicable Similar Laws;
(c): whether the investment will result in recognition of unrelated business taxable income by the plan and, if so, the potential after-tax investment return. For more information see "Material Tax Consequences" beginning on page 31 of this prospectus;
(d): whether the investment is made solely in the interests of the ERISA Plan participants;
(e): whether the investment would create any problems for the ERISA Plan's need for liquidity; and
(f): whether making the investment will comply with the delegation of control and prohibited transaction provisions of ERISA, the Code and any other applicable Similar Laws.

The person with investment discretion with respect to the assets of an employee benefit plan, often called a fiduciary, should determine whether an investment in our common units or debt securities is authorized by the appropriate governing instrument and is a proper investment for the plan. In addition, a fiduciary of an employee benefit plan may not deal with the plan's assets in his own interest, represent a person whose interests are adverse to the plan's in a transaction involving plan assets or receive any consideration from a third party in connection with a transaction involving plan assets. A violation of fiduciary requirements could result in liability for breach of fiduciary duty, disqualification from future fiduciary service, excise taxes and other adverse consequences to the ERISA Plan fiduciaries.

Table of Contents

Prohibited Transaction Issues

Section 406 of ERISA and Section 4975 of the Code prohibit employee benefit plans, and IRAs that are not considered part of an employee benefit plan, from engaging in specified transactions involving "plan assets" with parties that are "parties in interest" under ERISA or "disqualified persons" under the Code with respect to the plan, unless an exemption is available. A party in interest or disqualified person who engages in a non- exempt prohibited transaction may be subject to excise taxes and other penalties and liabilities under ERISA and the Code. In addition, the fiduciary of the ERISA Plan that engaged in such a non-exempt prohibited transaction may be subject to penalties and liabilities under ERISA and the Code.

The acquisition and/or holding of debt securities by an ERISA Plan with respect to which we or the initial purchasers are considered a party in interest or a disqualified person, may constitute or result in a direct or indirect prohibited transaction under Section 406 of ERISA and/or Section 4975 of the Code, unless the debt securities are acquired and held in accordance with an applicable statutory, class or individual prohibited transaction exemption. In this regard, the U.S. Department of Labor has issued prohibited transaction class exemptions, or PTCEs, that may apply to the acquisition, holding and, if applicable, conversion of the debt securities. These class exemptions include, without limitation, PTCE 84-14 respecting transactions determined by independent qualified professional asset managers, PTCE 90-1 respecting insurance company pooled separate accounts, PTCE 91-38 respecting bank collective investment funds, PTCE 95-60 respecting life insurance company general accounts and PTCE 96-23 respecting transactions determined by in-house asset managers. In addition, Section 408(b)(17) of ERISA and Section 4975(d)(20) of the Code provide limited relief from the prohibited transaction provisions of ERISA and Section 4975 of the Code for certain transactions, provided that neither the issuer of the securities nor any of its affiliates (directly or indirectly) have or exercise any discretionary authority or control or render any investment advice with respect to the assets of any ERISA Plan involved in the transaction and provided further that the ERISA Plan pays no more than adequate consideration in connection with the transaction. There can be no assurance that all of the conditions of any such exemptions will be satisfied.

Because of the foregoing, our debt securities should not be purchased or held (or converted to equity securities, in the case of any convertible debt) by a person investing "plan assets" of any employee benefit plan, unless such purchase and holding (or conversion, if any) will not constitute a non-exempt prohibited transaction under ERISA or the Code or similar violation of any applicable Similar Laws.

Plan Asset Issues

In addition to considering whether the purchase of our common units or debt securities is a prohibited transaction, a fiduciary of an employee benefit plan should consider whether the plan will, by investing in us, be deemed to own an undivided interest in our assets, with the result that our general partner also would be a fiduciary of the plan and our operations would be subject to the regulatory restrictions of ERISA, including its prohibited transaction rules, as well as the prohibited transaction rules of the Code and any other applicable Similar Laws.

The Department of Labor regulations, including 29 C.F.R. Section 2510.3-101, provide guidance with respect to whether the assets of an entity in which employee benefit plans acquire equity interests would be deemed "plan assets" under certain circumstances. Under these regulations, an entity's assets would not be considered to be "plan assets" if, among other things,

(a): the equity interests acquired by employee benefit plans are publicly offered securities, i.e., the equity interests are widely held by 100 or more investors independent of the issuer and each other, freely transferable and registered pursuant to certain provisions of the federal securities laws,

Table of Contents

(b): the entity is an "operating company," that is, it is primarily engaged in the production or sale of a product or service other than the investment of capital either directly or through a majority owned subsidiary or subsidiaries, or
(c): there is no significant investment by benefit plan investors, which is defined to mean that less than 25% of the value of each class of equity interest, disregarding certain interests held by our general partner, its affiliates, and some other persons, is held by the employee benefit plans that are subject to party of Title I of ERISA (which excludes governmental plans, foreign plans and non-electing church plans) and/or section 4975 of the Code.

With respect to an investment in our common units, our assets should not be considered "plan assets" under these regulations because it is expected that the investment will satisfy the requirements in (a) and (b) above and may also satisfy the requirements in (c) above (although we do not monitor the level of benefit plan investors as required for compliance with (c)). With respect to an investment in our debt securities, our assets should not be considered "plan assets" under these regulations because such securities are not equity securities or, even if they are issued with a feature that allows their conversion to equity securities, the securities in which they will be convertible will satisfy the requirements in (a) and (b) above. However, although we do not intend for our assets to be deemed as "plan assets" under these regulations, we cannot provide assurances regarding this issue to any investor.

The foregoing discussion of issues arising for employee benefit plan investments under ERISA, the Code and Similar Laws was intended to provide a brief summary of certain issues that employee benefit plans should consider before investing in our common units or debt securities, and should not be construed as legal advice. Plan fiduciaries contemplating a purchase of common units are urged to consult with their own counsel regarding the consequences under ERISA, the Code and Similar Laws in light of the serious penalties imposed on persons who engage in prohibited transactions or other violations.

Representation

Accordingly, by acceptance of our common units or debt securities, each purchaser and subsequent transferee of the securities will be deemed to have represented and warranted that either (i) no portion of the assets used by such purchaser or transferee to acquire and hold the securities constitutes assets of any employee benefit plan or (ii) the purchase and holding (and any conversion, if applicable) of the securities by such purchaser or transferee will not constitute a non-exempt prohibited transaction under Section 406 of ERISA or Section 4975 of the Code or similar violation under any applicable Similar Laws.

Table of Contents

PLAN OF DISTRIBUTION

Under this prospectus, we intend to offer our securities to the public:

•: through one or more broker-dealers;
•: through underwriters; or
•: directly to investors.

We will fix a price or prices of our securities at:

•: market prices prevailing at the time of any sale under this registration statement;
•: prices related to market prices; or
•: negotiated prices.

We may change the price of the securities offered from time to time.

We will pay or allow distributors' or sellers' commissions that will not exceed those customary in the types of transactions involved. Broker-dealers may act as agent or may purchase securities as principal and thereafter resell the securities from time to time:

•: in or through one or more transactions (which may involve crosses and block transactions) or distributions;
•: on the New York Stock Exchange;
•: in the over-the-counter market; or
•: in private transactions.

Broker-dealers or underwriters may receive compensation in the form of underwriting discounts or commissions and may receive commissions from purchasers of the securities for whom they may act as agents. If any broker-dealer purchases the securities as principal, it may effect resales of the securities from time to time to or through other broker-dealers, and other broker-dealers may receive compensation in the form of concessions or commissions from the purchasers of securities for whom they may act as agents.

To the extent required, the names of the specific managing underwriter or underwriters, if any, as well as other important information, will be set forth in prospectus supplements. In that event, the discounts and commissions we will allow or pay to the underwriters, if any, and the discounts and commissions the underwriters may allow or pay to dealers or agents, if any, will be set forth in, or may be calculated from, the prospectus supplements. Any underwriters, brokers, dealers and agents who participate in any sale of the securities may also engage in transactions with, or perform services for, us or our affiliates in the ordinary course of their businesses. We may indemnify underwriters, brokers, dealers and agents against specific liabilities, including liabilities under the Securities Act.

Offers to purchase securities may be solicited directly by us and the sale thereof may be made by us directly to institutional investors or others, who may be deemed to be underwriters within the meaning of the Securities Act of 1933 with respect to any resale thereof. The terms of any such sales will be described in the prospectus supplement relating thereto.

We may offer our common units into an existing trading market on the terms described in the prospectus supplement relating thereto. Underwriters and dealers who may participate in any at-the-market offerings will be described in the prospectus supplement relating thereto.

Table of Contents

The aggregate maximum compensation the underwriters will receive in connection with the sale of any securities under this prospectus and the registration statement of which it forms a part will not exceed 10% of the gross proceeds from the sale.

Because Financial Industry Regulatory Authority, Inc. ("FINRA") views our common units as interests in a direct participation program, any offering of common units under the registration statement of which this prospectus forms a part will be made in compliance with Rule 2310 of the FINRA Rules.

To the extent required, this prospectus may be amended or supplemented from time to time to describe a specific plan of distribution. The place and time of delivery for the securities in respect of which this prospectus is delivered will be set forth in the accompanying prospectus supplement.

In connection with offerings under this shelf registration and in compliance with applicable law, underwriters, brokers or dealers may engage in transactions which stabilize or maintain the market price of the securities at levels above those which might otherwise prevail in the open market. Specifically, underwriters, brokers or dealers may over-allot in connection with offerings, creating a short position in the securities for their own accounts. For the purpose of covering a syndicate short position or stabilizing the price of the securities, the underwriters, brokers or dealers may place bids for the securities or effect purchases of the securities in the open market. Finally, the underwriters may impose a penalty whereby selling concessions allowed to syndicate members or other brokers or dealers for distribution the securities in offerings may be reclaimed by the syndicate if the syndicate repurchases previously distributed securities in transactions to cover short positions, in stabilization transactions or otherwise. These activities may stabilize, maintain or otherwise affect the market price of the securities, which may be higher than the price that might otherwise prevail in the open market, and, if commenced, may be discontinued at any time.

Table of Contents

VALIDITY OF THE SECURITIES

In connection with particular offerings of the securities in the future, and if stated in the applicable prospectus supplement, the validity of those securities may be passed upon for us by Vinson & Elkins L.L.P. and for any underwriters or agents by counsel named in the applicable prospectus supplement. Members of Vinson & Elkins L.L.P. involved in this offering own an aggregate of approximately 2,550 of our common units.

EXPERTS

The financial statements incorporated in this registration statement by reference from the Partnership's Annual Report on Form 10-K for the year ended December 31, 2011 and the effectiveness of MarkWest Energy Partners, L.P.'s internal control over financial reporting as of December 31, 2011 have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports, which are incorporated herein by reference. Such financial statements have been so incorporated in reliance upon the reports of such firm given upon their authority as experts in accounting and auditing.

Table of Contents

$1,200,000,000

4.875% Senior Notes due 2025

PROSPECTUS SUPPLEMENT

May 28, 2015

Joint Book-Running Managers

Wells Fargo Securities

Barclays

BofA Merrill Lynch

Citigroup

Goldman, Sachs & Co.

J.P. Morgan

Morgan Stanley

RBC Capital Markets

SunTrust Robinson Humphrey

UBS Investment Bank

US Bancorp

Senior Co-Managers

BBVA

BNP PARIBAS

Capital One Securities

Comerica Securities

Natixis

PNC Capital Markets LLC

SMBC Nikko

MarkWest Energy Finance Inactive 424B5Prospectus supplement for primary offering