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                                                                 Exhibit (a)(5)


                           OFFER TO PURCHASE FOR CASH

                       16,730,502 SHARES OF COMMON STOCK
           (INCLUDING THE ASSOCIATED PREFERRED STOCK PURCHASE RIGHTS)

                                       OF

                         BARRETT RESOURCES CORPORATION
                                       AT

                              $73.00 NET PER SHARE

                                       BY

                          RESOURCES ACQUISITION CORP.,
                          A WHOLLY-OWNED SUBSIDIARY OF

                          THE WILLIAMS COMPANIES, INC.

     THE OFFER, PRORATION PERIOD AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00
MIDNIGHT, EASTERN TIME, ON MONDAY, JUNE 11, 2001, UNLESS THE OFFER IS EXTENDED.

                                                                    May 14, 2001

To Our Clients:

      Enclosed for your consideration are the Offer to Purchase dated May 14,
2001 (the "Offer to Purchase") and the related Letter of Transmittal (which, as
amended or supplemented from time to time, together constitute the "Offer") in
connection with the offer by Resources Acquisition Corp., a Delaware corporation
("Purchaser") and a wholly-owned subsidiary of The Williams Companies, Inc., a
Delaware corporation ("Williams"), to purchase 16,730,502 shares of common
stock, par value $0.01 per share (including the associated Rights (as defined
below), the "Shares") of Barrett Resources Corporation, a Delaware corporation
("Barrett Resources"), at a purchase price of $73.00 per Share, net to the
seller in cash, without interest, upon the terms and subject to the conditions
set forth in the Offer to Purchase. As used herein, "Rights" means the preferred
stock purchase rights associated with the Shares and issued under the Rights
Agreement (the "Rights Agreement") dated as of August 5, 1997 by and between
Barrett Resources and Fleet National Bank, as successor to BankBoston, N.A., as
Rights Agent, as amended. Unless the content otherwise requires, all references
to the shares shall be deemed to include the Rights, and all references to the
Rights shall be deemed to include the benefits that may inure to holders of the
Rights pursuant to the Rights Agreement.

      If a stockholder desires to tender Shares pursuant to the Offer and such
stockholder's Share certificates are not immediately available or time will not
permit all required documents to reach the Depositary prior to the Expiration
Date (as defined in the Offer to Purchase) or the procedures for book-entry
transfer cannot be completed on a timely basis, such Shares may nevertheless be
tendered according to the guaranteed delivery procedures set forth in Section 3
of the Offer to Purchase. See Instruction 2 of the Letter of Transmittal.
Delivery of documents to the Book-Entry Transfer Facility (as defined in the
Offer to Purchase) in accordance with the Book-Entry Transfer Facility's
procedures does not constitute delivery to the Depositary.

      WE ARE THE HOLDER OF RECORD OF SHARES HELD BY US FOR YOUR ACCOUNT. A
TENDER OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF RECORD AND
PURSUANT TO YOUR INSTRUCTIONS. THE ENCLOSED LETTER OF TRANSMITTAL IS FURNISHED
TO YOU FOR YOUR INFORMATION ONLY AND CANNOT BE USED BY YOU TO TENDER SHARES HELD
BY US FOR YOUR ACCOUNT.
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      ACCORDINGLY, WE REQUEST INSTRUCTIONS AS TO WHETHER YOU WISH TO HAVE US
TENDER ON YOUR BEHALF ANY OR ALL SHARES HELD BY US FOR YOUR ACCOUNT PURSUANT TO
THE TERMS AND CONDITIONS SET FORTH IN THE OFFER.

      Please note the following:

             1. The tender price is $73.00 per Share, net to you in cash,
      without interest, upon the terms and subject to the conditions set forth
      in the Offer.

             2. The Offer is being made for 16,730,502 Shares (representing
      approximately 50% of the outstanding Shares). If more than 16,730,502
      Shares are tendered, Purchaser will purchase 16,730,502 of such tendered
      Shares on a pro rata basis.

             3. The Board of Directors of Barrett Resources has unanimously
      approved the Offer, the Merger (as defined below), and the Merger
      Agreement (as defined below), determined that the terms of each are
      advisable, fair to, and in the best interests of, the stockholders of
      Barrett Resources, and recommends that Barrett Resources' stockholders
      accept the Offer and tender their Shares pursuant to the Offer and approve
      the Merger at the time of Barrett Resources' stockholder meeting.

             4. The Offer is being made pursuant to an Agreement and Plan of
      Merger, dated as of May 7, 2001 (the "Merger Agreement"), by and among
      Williams, Purchaser and Barrett Resources, pursuant to which, following
      the consummation of the Offer and in accordance with the Delaware General
      Corporation Law, and subject to the satisfaction or waiver of certain
      conditions, including approval of stockholders of Barrett Resources,
      Barrett Resources will merge with and into Purchaser with Purchaser
      continuing as the surviving corporation, unless certain conditions related
      to taxes are not satisfied, in which case Purchaser will merge with and
      into Barrett Resources with Barrett Resources as the surviving corporation
      (collectively, the "Merger"). In the Merger, stockholders of Barrett
      Resources will exchange each of their Shares for 1.767 shares of Williams
      common stock, par value $1.00 per share, with cash in lieu of fractional
      shares. The exchange ratio of 1.767 shares of Williams common stock per
      Share is a fixed ratio that will not be adjusted as a result of any
      increase or decrease in the market price of either shares of Williams
      common stock or Shares of Barrett Resources. The market price of shares of
      Williams common stock at the time the Merger is completed may therefore be
      higher or lower than their price on the date of this document, the date of
      the consummation of the Offer and on the date of the special meeting of
      stockholders to be held to vote on the Merger. As a result, the Williams
      common stock you as a Barrett Resources stockholder may receive in the
      Merger may be worth more or less than the price being offered for the
      Shares in the Offer. WE URGE YOU TO OBTAIN CURRENT MARKET QUOTATIONS FOR
      BOTH SHARES OF BARRETT RESOURCES AND SHARES OF WILLIAMS COMMON STOCK
      BEFORE DECIDING WHETHER TO TENDER YOUR SHARES. Section 11 of the Offer to
      Purchase contains a more detailed description of the Merger Agreement, the
      Offer, the Merger and the consideration payable in the Merger in respect
      of the Shares.

             5. The Offer is conditioned upon, among other things, (i) there
      being validly tendered and not properly withdrawn prior to the Expiration
      Date of the Offer at least 16,730,502 Shares and (ii) the expiration or
      termination of the waiting periods under the Hart-Scott-Rodino Antitrust
      Improvements Act of 1976, as amended, and the rules and regulations
      promulgated thereunder. The Offer is not subject to a financing condition.
      The Offer is also subject to the other conditions set forth in the Offer
      to Purchase. See the Introduction and Section 14 of the Offer to Purchase.

             6. Any stock transfer taxes applicable to the sale of Shares to
      Purchaser pursuant to the Offer will be paid by Purchaser, except as
      otherwise provided in Instruction 6 of the Letter of Transmittal.

             7. The Offer, proration period and withdrawal rights will expire at
      12:00 Midnight, eastern time, on Monday, June 11, 2001, unless the Offer
      is extended.

             8. Payment for Shares purchased pursuant to the Offer will in all
      cases be made only after timely receipt by the Depositary of (a) Share
      Certificates or timely confirmation of the book-entry transfer of such
      Shares into the account maintained by the Book-Entry Transfer Facility (as
      defined in the Offer to Purchase), pursuant to the procedures set forth in
      Section 3 of the Offer to Purchase, (b) the Letter of Transmittal (or a
      facsimile thereof), properly completed and duly executed, with any
      required signature guarantees or an Agent's Message (as defined in the
      Offer to Purchase), in connection with a book-entry delivery and (c) any
      other

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      documents required by the Letter of Transmittal. Accordingly, payment may
      not be made to all tendering stockholders at the same time, depending upon
      when Share Certificates or confirmations of book-entry transfer of such
      Shares into the Depositary's account at the Book-Entry Transfer Facility
      are actually received by the Depositary.

      If you wish to have us tender any or all of the Shares held by us for your
account, please so instruct us by completing, executing, detaching and returning
to us the instruction form set forth on the back page of this letter. If you
authorize the tender of your Shares, all such Shares will be tendered unless
otherwise specified on the back page of this letter. An envelope to return your
instructions to us is enclosed. YOUR INSTRUCTIONS SHOULD BE FORWARDED TO US IN
AMPLE TIME TO PERMIT US TO SUBMIT A TENDER ON YOUR BEHALF PRIOR TO THE
EXPIRATION OF THE OFFER.

      Purchaser is not aware of any state where making the Offer is prohibited
by any applicable law. If Purchaser becomes aware of any applicable law
prohibiting the making of the Offer or the acceptance of the Shares pursuant
thereto, Purchaser will make a good faith effort to comply with such law or seek
to have such law declared inapplicable to the Offer. If, after such good faith
effort, Purchaser cannot comply with such state statute, the Offer will not be
made (nor will tenders be accepted from or on behalf of) holders of Shares in
such state. In any jurisdiction where the securities, blue sky or other laws
require the Offer to be made by a licensed broker or dealer, the Offer will be
deemed to be made on behalf of the Purchaser by Merrill Lynch & Co. (the Dealer
Manager), or by one or more registered brokers or dealers that are licensed
under the laws of such jurisdiction.

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          INSTRUCTIONS WITH RESPECT TO THE OFFER TO PURCHASE FOR CASH

                       16,730,502 SHARES OF COMMON STOCK
           (INCLUDING THE ASSOCIATED PREFERRED STOCK PURCHASE RIGHTS)

                                       OF

                         BARRETT RESOURCES CORPORATION
                                       AT

                              $73.00 NET PER SHARE

                                       BY

                          RESOURCES ACQUISITION CORP.,
                          A WHOLLY-OWNED SUBSIDIARY OF

                          THE WILLIAMS COMPANIES, INC.

      The undersigned acknowledge(s) receipt of your letter, the enclosed Offer
to Purchase dated May 14, 2001 (the "Offer to Purchase") and the related Letter
of Transmittal (which, as amended or supplemented from time to time, together
constitute the "Offer") in connection with Purchaser's offer to purchase
16,730,502 shares of common stock, par value $0.01 per share (including the
associated preferred stock purchase rights, the "Shares") of Barret Resources
Corporation, a Delaware corporation.

      This will instruct you to tender the number of Shares indicated below (or
if no number is indicated below, all Shares) which are held by you for the
account of the undersigned, upon the terms and subject to the conditions set
forth in the Offer.

Number of Shares to be Tendered:
- ------------------------------ Shares*

                                   SIGN BELOW

Account Number:
- ------------------------------  Signature(s)
- ------------------------------------------------------

Dated:
- ------------------------------------, 2001

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                          PLEASE TYPE OR PRINT NAME(S)

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                     PLEASE TYPE OR PRINT ADDRESS(ES) HERE

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                       AREA CODE AND TELEPHONE NUMBER(S)

- --------------------------------------------------------------------------------
              TAXPAYER IDENTIFICATION OR SOCIAL SECURITY NUMBER(S)

- ---------------
* Unless otherwise indicated, it will be assumed that you instruct us to tender
  all Shares held by us for your account.

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