====================================
                                          
                                          
                            FIRST SUPPLEMENTAL INDENTURE
                                          
                            Dated as of _______ __, 1998
                                          
                                      between
                                          
                         PUBLIC SERVICE COMPANY OF COLORADO
                                          
                                     AS ISSUER
                                          
                                        and
                                          
                                THE BANK OF NEW YORK
                                          
                                     AS TRUSTEE
                                          
                                          
                       =====================================
                                           
                                          


                                  TABLE OF CONTENTS


                                                                                 Page
                                                                                 ----
                                                                              
I    DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
     1.1. Definition of Terms. . . . . . . . . . . . . . . . . . . . . . . . . . . .2

II   GENERAL TERMS AND CONDITIONS OF THE DEBENTURES. . . . . . . . . . . . . . . . .3
     2.1. Designation and Principal Amount . . . . . . . . . . . . . . . . . . . . .3
     2.2. Maturity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
     2.3. Form and Payment . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
     2.4. Global Debenture . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
     2.5. Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

III  REDEMPTION OF THE DEBENTURES. . . . . . . . . . . . . . . . . . . . . . . . . .6
     3.1. Special Event Redemption . . . . . . . . . . . . . . . . . . . . . . . . .6
     3.2. Optional Redemption by Company . . . . . . . . . . . . . . . . . . . . . .7
     3.3. No Sinking Fund. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

IV   EXTENSION OF INTEREST PAYMENT PERIOD. . . . . . . . . . . . . . . . . . . . . .8
     4.1. Extension of Interest Payment Period . . . . . . . . . . . . . . . . . . .8
     4.2. Notice of Extension. . . . . . . . . . . . . . . . . . . . . . . . . . . .9
     4.3. Limitation of Transactions . . . . . . . . . . . . . . . . . . . . . . . .9

V    EXPENSES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1. Payment of Expenses. . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.2. Payment Upon Resignation or Removal. . . . . . . . . . . . . . . . . . . 10

VI   SUBORDINATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1. Subordination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

VII  COVENANT TO LIST ON EXCHANGE. . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1. Listing on an Exchange . . . . . . . . . . . . . . . . . . . . . . . . . 11

VIII FORM OF DEBENTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1. Form of Debenture. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

IX   ORIGINAL ISSUE OF DEBENTURES. . . . . . . . . . . . . . . . . . . . . . . . . 20
     9.1. Original Issue of Debentures . . . . . . . . . . . . . . . . . . . . . . 20

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X    MISCELLANEOUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.1.     Ratification of Indenture . . . . . . . . . . . . . . . . . . . . . 20
     10.2.     Trustee Not Responsible for Recitals. . . . . . . . . . . . . . . . 20
     10.3.     Governing Law . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.4.     Separability. . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.5.     Counterparts. . . . . . . . . . . . . . . . . . . . . . . . . . . . 21


                                       ii



          FIRST SUPPLEMENTAL INDENTURE, dated as of ____________, 1998 (the 
"First Supplemental Indenture"), between Public Service Company of Colorado, 
a corporation duly organized and existing under the laws of the State of 
Colorado, having its principal office at 1225 17th Street, Denver, Colorado 
80202 (the "Company"), and The Bank of New York, as trustee (the "Trustee") 
under the Indenture dated as of __________, 1998 (the "Indenture"), between 
the Company and the Trustee.

          WHEREAS, the Company executed and delivered the Indenture to the 
Trustee to provide for the future issuance of the Company's unsecured 
debentures, notes or other evidence of indebtedness (the "Securities"), to be 
issued from time to time in one or more series as might be determined by the 
Company under the Indenture;

          WHEREAS, pursuant to the terms of the Indenture, the Company 
desires to provide for the establishment of a new series of its Securities to 
be known as its ___% Deferrable Interest Subordinated Debentures due 
_________  (the "Debentures"), the form and substance of such Debentures and 
the terms, provisions and conditions thereof to be set forth as provided in 
the Indenture and this First Supplemental Indenture;

          WHEREAS, PSCO Capital Trust I, a Delaware statutory business trust 
(the "Trust"), intends to offer to the public $___ million aggregate 
liquidation amount of its ___% Trust Originated Preferred Securities (the 
"Preferred Securities"), representing undivided beneficial interests in the 
assets of the Trust, and proposes to invest the proceeds from such offering, 
together with the proceeds of the issuance and sale by the Trust to the 
Company of $___ million aggregate liquidation amount of its ___% Trust 
Originated Common Securities, in $____ million aggregate principal amount of 
the Debentures; and 

          WHEREAS, the Company has requested that the Trustee execute and 
deliver this First Supplemental Indenture, and all requirements necessary to 
make this First Supplemental Indenture a valid instrument in accordance with 
its terms, and to make the Debentures, when executed by the Company and 
authenticated and delivered by the Trustee, the valid obligations of the 
Company, have been performed, and the execution and delivery of this First 
Supplemental Indenture has been duly authorized in all respects.

          NOW THEREFORE, in consideration of the purchase and acceptance of 
the Debentures by the Holders thereof, and for the purpose of setting forth, 
as provided in the Indenture, the form and substance of the Debentures and 
the terms, provisions and conditions thereof, the Company covenants and 
agrees with the Trustee as follows:

                                       



                                     ARTICLE I
                                    DEFINITIONS

SECTION 1.1.   DEFINITION OF TERMS.

          Unless the context otherwise requires:

          (a)  a term defined in the Indenture has the same meaning when used 
in this First Supplemental Indenture;

          (b)  a term defined anywhere in this First Supplemental Indenture 
has the same meaning throughout;

          (c)  the singular includes the plural and vice versa;

          (d)  headings are for convenience of reference only and do not 
affect interpretation;

          (e)  the following terms have the meanings given to them in the 
Declaration:  (i) Administrative Trustee; (ii) Business Day; (iii) Clearing 
Agency; (iv) Delaware Trustee; (v) Guarantee; (vi) Preferred Securities 
Certificate; (vii) Pricing Agreement; (viii) Property Trustee; (ix) Trust 
Securities; and (x) Underwriting Agreement; and

          (f)  the following terms have the meanings given to them in this 
Section 1.1(f):

          "Additional Interest" shall have the meaning set forth in Section 
2.5(c).

          "Change in 1940 Act Law" shall have the meaning set forth in 
Section 3.1.

          "Compounded Interest" shall have the meaning set forth in Section 
4.1.

          "Coupon Rate" shall have the meaning set forth in Section 2.5(a).

          "Declaration" means the Amended and Restated Declaration of Trust 
of PSCO Capital Trust I, a Delaware statutory business trust, dated as of 
_________, 1998.

          "Deferred Interest" shall have the meaning set forth in Section 4.1.

          "Dissolution Event" means that the Trust is to be dissolved in 
accordance with the Declaration, and the Debentures held by the Property 
Trustee are to be distributed to the holders of the Trust Securities issued 
by the Trust pro rata in accordance with the Declaration.

          "Extended Interest Payment Period" shall have the meaning set forth 
in Section 4.1.

          "Global Debenture" shall have the meaning set forth in Section 
2.4(a).

                                       2



          "Interest Payment Date" shall have the meaning set forth in Section 
2.5(a).

          "Investment Company Event" shall have the meaning set forth in 
Section 3.1.

          "Maturity Date" means the date on which the Debentures mature and 
on which the principal shall be due and payable together with all accrued and 
unpaid interest thereon including Compounded Interest and Additional 
Interest, if any.

          "Ministerial Action" shall have the meaning set forth in Section 
3.1.

          "90 Day Period" shall have the meaning set forth in Section 3.1.

          "Non Book-Entry Preferred Securities" shall have the meaning set 
forth in Section 2.4(a).

          "Optional Redemption Price" shall have the meaning set forth in 
Section 3.2(a).

          "Redemption Price" shall have the meaning set forth in Section 3.1.

          "Special Event" shall have the meaning set forth in Section 3.1.

          "Tax Event" shall have the meaning set forth in Section 3.1.     

                                      ARTICLE II
                    GENERAL TERMS AND CONDITIONS OF THE DEBENTURES

SECTION 2.1.   DESIGNATION AND PRINCIPAL AMOUNT.

          There is hereby authorized a series of Securities designated the 
"___% Deferrable Interest Subordinated Debentures due _____ ", limited in 
aggregate principal amount to $___ million, which amount shall be as set 
forth in any written order of the Company for the authentication and delivery 
of Debentures pursuant to Section 303 of the Indenture.

SECTION 2.2.   MATURITY.

          The Maturity Date of the Debentures is ____________, 20__.

SECTION 2.3.   FORM AND PAYMENT.

          Except as provided in Section 2.4, the Debentures shall be issued 
in fully registered certificated form without interest coupons in 
denominations of $25 or integral multiples of $25.  Principal and interest on 
the Debentures issued in certificated form will be payable, the transfer of 
such Debentures will be registrable and such Debentures will be exchangeable 
for Debentures 

                                       3



bearing identical terms and provisions at the office or agency of the 
Trustee; provided, however, that payment of interest may be made at the 
option of the Company by check mailed to the Holder at such address as shall 
appear in the Security Register.  Notwithstanding the foregoing, so long as 
the Holder of any Debentures is the Property Trustee, the payment of the 
principal of and interest (including Compounded Interest and Additional 
Interest, if any) on such Debentures held by the Property Trustee will be 
made at such place and to such account as may be designated by the Property 
Trustee.

SECTION 2.4.   GLOBAL DEBENTURE.

          (a)  In connection with a Dissolution Event,

               (i) the Debentures in certificated form may be presented to the
     Trustee by the Property Trustee in exchange for a global Debenture in an
     aggregate principal amount equal to the aggregate principal amount of all
     outstanding Debentures (a "Global Debenture"), to be registered in the name
     of the Depository, or its nominee, and delivered by the Trustee to the
     Depository for crediting to the accounts of its participants pursuant to
     the instructions of the Administrative Trustee.  The Company upon any such
     presentation shall execute a Global Debenture in such aggregate principal
     amount and deliver the same to the Trustee for authentication and delivery
     in accordance with the Indenture and this First Supplemental Indenture. 
     Payments on the Debentures issued as a Global Debenture will be made to the
     Depository; and

               (ii) if any Preferred Securities are held in non book-entry
     certificated form, the Debentures in certificated form may be presented to
     the Trustee by the Property Trustee and any Preferred Security Certificate
     which represents Preferred Securities other than Preferred Securities held
     by the Clearing Agency or its nominee ("Non Book-Entry Preferred
     Securities") will be deemed to represent beneficial interests in Debentures
     presented to the Trustee by the Property Trustee having an aggregate
     principal amount equal to the aggregate liquidation amount of the Non
     Book-Entry Preferred Securities until such Preferred Security Certificates
     are presented to the Security Registrar for transfer or reissuance at which
     time such Preferred Security Certificates will be cancelled and a
     Debenture, registered in the name of the holder of the Preferred Security
     Certificate or the transferee of the holder of such Preferred Security
     Certificate, as the case may be, with an aggregate principal amount equal
     to the aggregate liquidation amount of the Preferred Security Certificate
     cancelled, will be executed by the Company and delivered to the Trustee for
     authentication and delivery in accordance with the Indenture and this First
     Supplemental Indenture.  On issue of such Debentures, Debentures with an
     equivalent aggregate principal amount that were presented by the Property
     Trustee to the Trustee will be deemed to have been cancelled.

                                       4



          (b)  Unless and until it is exchanged for the Debentures in 
registered form, a Global Debenture may be transferred, in whole but not in 
part, only to another nominee of the Depository, or to a successor Depository 
selected or approved by the Company or to a nominee of such successor 
Depository.

          (c)  If at any time the Depository notifies the Company that it is 
unwilling or unable to continue as Depository or if at any time the 
Depository for such series shall no longer be registered or in good standing 
under the Securities Exchange Act of 1934, as amended, or other applicable 
statute or regulation, and a successor Depository for such series is not 
appointed by the Company within 90 days after the Company receives such 
notice or becomes aware of such condition, as the case may be, the Company 
will execute, and, subject to Article Three of the Indenture, the Trustee, 
upon written notice from the Company, will authenticate and deliver the 
Debentures in definitive registered form without coupons, in authorized 
denominations, and in an aggregate principal amount equal to the principal 
amount of the Global Debenture in exchange for such Global Debenture.  In 
addition, the Company may at any time determine that the Debentures shall no 
longer be represented by the Global Debenture.  In such event the Company 
will execute, and subject to Section 303 of the Indenture, the Trustee, upon 
receipt of an Officers' Certificate evidencing such determination by the 
Company, will authenticate and deliver the Debentures in definitive 
registered form without coupons, in authorized denominations, and in an 
aggregate principal amount equal to the principal amount of the Global 
Debenture in exchange for such Global Debenture.  Upon the exchange of the 
Global Debenture for such Debentures in definitive registered form without 
coupons, in authorized denominations, the Global Debenture shall be cancelled 
by the Trustee.  Such Debentures in definitive registered form issued in 
exchange for the Global Debenture shall be registered in such names and in 
such authorized denominations as the Depository, pursuant to instructions 
from its direct or indirect participants or otherwise, shall instruct the 
Trustee.  The Trustee shall deliver such Debentures to the Depository for 
delivery to the Persons in whose names such Debentures are so registered.

SECTION 2.5.   INTEREST.

          (a)  Each Debenture will bear interest at the rate of ___% per 
annum (the "Coupon Rate") from the original date of issuance until the 
principal thereof becomes due and payable, and on any overdue principal and 
(to the extent that payment of such interest is enforceable under applicable 
law) on any overdue installment of interest at the Coupon Rate, compounded 
quarterly, payable (subject to the provisions of Article IV) quarterly in 
arrears on March 31, June 30, September 30 and December 31 of each year 
(each, an "Interest Payment Date," commencing on June 30, 1998), to the 
Person in whose name such Debenture or any predecessor Debenture is 
registered, at the close of business on the regular record date for such 
interest installment, which, in respect of (i) Debentures of which the 
Property Trustee is the Holder and the Preferred Securities are in book-entry 
only form or (ii) a Global Debenture, shall be the close of business on the 
Business Day next preceding that Interest Payment Date. Notwithstanding the 
foregoing sentence, if (i) the Debentures are held by the Property Trustee 
and the Preferred Securities are no longer in book-entry only form or (ii) 
the Debentures are not represented by a Global Debenture, the Company may 
select a regular record date for such interest 

                                       5



installment which shall be any date at least one Business Day but less than 
sixty business days before an Interest Payment Date.

          (b)  The amount of interest payable for any period will be computed 
on the basis of a 360-day year of twelve 30-day months.  Except as provided 
in the following sentence, the amount of interest payable for any period 
shorter than a full quarterly period for which interest is computed, will be 
computed on the basis of the actual number of days elapsed in such a 90-day 
period.  In the event that any date on which interest is payable on the 
Debentures is not a Business Day, then payment of interest payable on such 
date will be made on the next succeeding day which is a Business Day (and 
without any interest or other payment in respect of any such delay), except 
that, if such Business Day is in the next succeeding calendar year, such 
payment shall be made on the immediately preceding Business Day, in each case 
with the same force and effect as if made on such date.

          (c)  If, at any time while the Property Trustee is the Holder of 
any Debentures, the Trust or the Property Trustee is required to pay any 
taxes, duties, assessments or governmental charges of whatever nature (other 
than withholding taxes) imposed by the United States, or any other taxing 
authority, then, in any case, the Company will pay as additional interest 
("Additional Interest") on the Debentures held by the Property Trustee, such 
additional amounts as shall be required so that the net amounts received and 
retained by the Trust and the Property Trustee after paying such taxes, 
duties, assessments or other governmental charges will be equal to the 
amounts the Trust and the Property Trustee would have received had no such 
taxes, duties, assessments or other government charges been imposed.

                                     ARTICLE III
                             REDEMPTION OF THE DEBENTURES

SECTION 3.1.   SPECIAL EVENT REDEMPTION.

          If a Special Event (as defined below) has occurred and is 
continuing then, notwithstanding Section 3.2(a) but subject to Section 
3.2(b), the Company shall have the right upon not less than 30 days nor more 
than 60 days notice to the Holders of the Debentures to redeem the 
Debentures, in whole or in part, for cash within 90 days following the 
occurrence of such Special Event (the "90 Day Period") at a redemption price 
equal to 100% of the principal amount to be redeemed plus any accrued and 
unpaid interest thereon, including Compounded Interest and Additional 
Interest, if any, to the date of such redemption (the "Redemption Price"); 
provided, however, that in the case of an occurrence of a Tax Event, if at 
the time there is available to the Company the opportunity to eliminate, 
within the 90 Day Period, the Tax Event by taking some ministerial action 
("Ministerial Action"), such as filing a form or making an election, or 
pursuing some other similar reasonable measure which has no adverse effect on 
the Company, the Trust or the Holders of the Trust Securities issued by the 
Trust, the Company shall pursue such Ministerial Action in lieu of 
redemption, and, provided, further, that the Company shall have no right to 
redeem the Debentures while the Trust is pursuing any Ministerial Action 

                                       6



pursuant to its obligations under the Declaration.  The Redemption Price 
shall be paid prior to 12:00 noon, New York time, on the date of such 
redemption or at such earlier time as the Company determines, provided that 
the Company shall deposit with the Trustee an amount sufficient to pay the 
Redemption Price by 10:00 a.m., New York time, on the date such Redemption 
Price is to be paid.

          A "Special Event" shall mean either a Tax Event or an Investment 
Company Event.  "Tax Event" shall mean that the Company shall have received 
an opinion of counsel (which may be regular counsel to the Company or an 
Affiliate, but not an employee thereof, which must be acceptable to the 
Property Trustee of the Trust) experienced in such matters to the effect 
that, as a result of any amendment to, or change (including any announced 
prospective change) in, the laws (or any regulations thereunder) of the 
United States or any political subdivision or taxing authority thereof or 
therein affecting taxation, or as a result of any official administrative 
pronouncement or judicial decision interpreting or applying such laws or 
regulations, which amendment or change is effective or such interpretation or 
pronouncement is announced on or after the date of original issuance of 
Preferred Securities, there is more than an insubstantial risk that (i) the 
Trust is subject to United States Federal income tax with respect to interest 
received on the Debentures, (ii) interest payable by the Company to the Trust 
on the Debentures will not be deductible for United States Federal income tax 
purposes, or (iii) the Trust is subject to more than a de minimis amount of 
other taxes, duties, assessments or other governmental charges.  "Investment 
Company Event" shall mean the occurrence of a change in law or regulation or 
a change in interpretation or application of law or regulation by any 
legislative body, court, governmental agency or regulatory authority (a 
"Change in 1940 Act Law") to the effect that the Trust is or will be 
considered an "Investment Company" that is required to be registered under 
the Investment Company Act of 1940, as amended, which Change in 1940 Act Law 
becomes effective on or after the date of original issuance of the Preferred 
Securities.  

SECTION 3.2.   OPTIONAL REDEMPTION BY COMPANY.

          (a)  Subject to the provisions of Section 3.2(b) and to the 
provisions of Article Eleven of the Indenture, except as otherwise may be 
specified in this First Supplemental Indenture, the Company shall have the 
right to redeem the Debentures, in whole or in part, from time to time, on or 
after _________, ____, at a redemption price equal to 100% of the principal 
amount to be redeemed plus any accrued and unpaid interest thereon, including 
Compounded Interest and Additional Interest, if any, to the date of such 
redemption (the "Optional Redemption Price").  Any redemption pursuant to 
this paragraph will be made upon not less than 30 days nor more than 60 days 
notice to the Holder of the Debentures, at the Optional Redemption Price.  If 
the Debentures are only partially redeemed pursuant to this Section 3.2, the 
Debentures will be redeemed pro rata or by lot or by any other method 
utilized by the Trustee; provided, that if at the time of redemption the 
Debentures are registered as a Global Debenture, the Depository shall 
determine, in accordance with its procedures, the principal amount of such 
Debentures held by each Debenture Holder to be redeemed.  The Optional 
Redemption Price shall be paid prior to 12:00 noon, New York time, on the 
date of such redemption or at such earlier time as the Company determines, 
provided that the Company shall deposit with the Trustee an amount 

                                       7



sufficient to pay the Optional Redemption Price by 10:00 a.m., New York time, 
on the date such Optional Redemption Price is to be paid.

          (b)  If a partial redemption of the Debentures would result in the 
delisting of the Preferred Securities issued by the Trust from any national 
securities exchange or other organization on which the Preferred Securities 
are then listed, the Company shall not be permitted to effect such partial 
redemption and may only redeem the Debentures in whole.

SECTION 3.3.   NO SINKING FUND.

     The Debentures are not entitled to the benefit of any sinking fund.

                                      ARTICLE IV
                         EXTENSION OF INTEREST PAYMENT PERIOD

SECTION 4.1.   EXTENSION OF INTEREST PAYMENT PERIOD.

          The Company shall have the right, at any time and from time to time 
during the term of the Debentures, so long as no Event of Default with 
respect to the Debentures has occurred and is continuing, to defer payments 
of interest by extending the interest payment period of such Debentures for a 
period not exceeding 20 consecutive quarters (the "Extended Interest Payment 
Period"), during which Extended Interest Payment Period no interest shall be 
due and payable; provided that no Extended Interest Payment Period may extend 
beyond the Maturity Date. To the extent permitted by applicable law, 
interest, the payment of which has been deferred because of the extension of 
the interest payment period pursuant to this Section 4.1, will bear interest 
thereon at the Coupon Rate compounded quarterly for each quarter of the 
Extended Interest Payment Period ("Compounded Interest").  At the end of the 
Extended Interest Payment Period, the Company shall pay all interest accrued 
and unpaid on the Debentures, including any Additional Interest and 
Compounded Interest (together, "Deferred Interest") that shall be payable to 
the Holders of the Debentures in whose names the Debentures are registered in 
the Security Register on the first record date after the end of the Extended 
Interest Payment Period.  Before the termination of any Extended Interest 
Payment Period, the Company may further extend such period, provided that 
such period together with all such further extensions thereof shall not 
exceed 20 consecutive quarters, or extend beyond the Maturity Date.  Upon the 
termination of any Extended Interest Payment Period and upon the payment of 
all Deferred Interest then due, the Company may commence a new Extended 
Interest Payment Period, subject to the foregoing requirements.  No interest 
shall be due and payable during an Extended Interest Payment Period, except 
(i) at the end thereof and (ii) upon a redemption of the Debentures during an 
Extended Interest Payment Period, but the Company may prepay at any time all 
or any portion of the interest accrued during an Extended Interest Payment 
Period.

                                       8



SECTION 4.2.   NOTICE OF EXTENSION.

          (a)  If the Property Trustee is the only registered Holder of the 
Debentures at the time the Company selects an Extended Interest Payment 
Period, the Company shall give written notice to the Administrative Trustee, 
the Property Trustee and the Trustee of its selection of such Extended 
Interest Payment Period one Business Day before the earlier of (i) the next 
succeeding date on which Distributions on the Preferred Securities issued by 
the Trust are payable, or (ii) the date the Trust is required to give notice 
of the record date, or the date such Distributions are payable, to the New 
York Stock Exchange or other applicable self-regulatory organization or to 
holders of the Preferred Securities issued by the Trust, but in any event at 
least one Business Day before such record date.

          (b)  If the Property Trustee is not the only Holder of the 
Debentures at the time the Company selects an Extended Interest Payment 
Period, the Company shall give the Holders of the Debentures and the Trustee 
written notice of its selection of such Extended Interest Payment Period at 
least ten Business Days before the earlier of (i) the next succeeding 
Interest Payment Date, or (ii) the date the Company is required to give 
notice of the record or payment date of such interest payment to the New York 
Stock Exchange or other applicable self-regulatory organization or to Holders 
of the Debentures. 

          (c)  The quarter in which any notice is given pursuant to 
paragraphs (a) or (b) of this Section 4.2 shall be counted as one of the 20 
quarters permitted in the maximum Extended Interest Payment Period permitted 
under Section 4.1.

SECTION 4.3.   LIMITATION OF TRANSACTIONS.

          If (i) the Company shall exercise its right to defer payment of 
interest as provided in Section 4.1, (ii) there shall have occurred any Event 
of Default, as defined in the Indenture, or (iii) the Company shall be in 
default with respect to its payment obligations under the Guarantee, then (a) 
the Company shall not declare or pay any dividend on, make any distributions 
with respect to, or redeem, purchase, acquire or make a liquidation payment 
with respect to, any of its capital stock (other than (i) as a result of a 
reclassification of its capital stock or the exchange or conversion of one 
class or series of its capital stock for another class or series of its 
capital stock or (ii) the purchase of fractional interests in shares of its 
capital stock pursuant to the conversion or exchange provisions of such 
capital stock or security being converted or exchanged) or make any guarantee 
payment with respect thereto, (b) the Company shall not make any payment of 
interest, principal or premium, if any, on or repay, repurchase or redeem any 
debt securities issued by the Company which rank PARI PASSU with or junior to 
the Debentures and (c) the Company shall not make any guarantee payments with 
respect to the foregoing (other than pursuant to the Guarantee and any 
similar guarantee issued by the Company on behalf of holders of preferred 
securities issued by an issuer holding Securities issued under the Indenture).

                                       9



                                      ARTICLE V
                                       EXPENSES

SECTION 5.1.   PAYMENT OF EXPENSES.

          In connection with the offering, sale and issuance of the 
Debentures to the Property Trustee and in connection with the sale of the 
Trust Securities by the Trust, the Company, in its capacity as borrower with 
respect to the Debentures, shall:

          (a)  pay all costs and expenses relating to the offering, sale and 
issuance of the Debentures, including commissions to the underwriters payable 
pursuant to the Underwriting Agreement and the Pricing Agreement and 
compensation of the Trustee under the Indenture in accordance with the 
provisions of Section 607 of the Indenture;

          (b)  pay all costs and expenses of the Trust (including, but not 
limited to, costs and expenses relating to the organization of the Trust, the 
offering, sale and issuance of the Trust Securities (including commissions to 
the underwriters in connection therewith), the fees and expenses of the 
Property Trustee and the Delaware Trustee, the costs and expenses relating to 
the operation of the Trust, including without limitation, costs and expenses 
of accountants, attorneys, statistical or bookkeeping services, expenses for 
printing and engraving and computing or accounting equipment, paying 
agent(s), registrar(s), transfer agent(s), duplicating, travel and telephone 
and other telecommunications expenses and costs and expenses incurred in 
connection with the acquisition, financing, and disposition of Trust assets); 

          (c)  be primarily liable for any indemnification obligations 
arising with respect to the Declaration; and

          (d)  pay any and all taxes (other than United States withholding 
taxes attributable to the Trust or its assets) and all liabilities, costs and 
expenses with respect to such taxes of the Trust.

SECTION 5.2.   PAYMENT UPON RESIGNATION OR REMOVAL.

          Upon termination of this First Supplemental Indenture or the 
Indenture or the removal or resignation of the Trustee pursuant to this 
Section 5.2, the Company shall pay to the Trustee all amounts accrued to the 
date of such termination, removal or resignation.  Upon termination of the 
Declaration or the removal or resignation of the Delaware Trustee or the 
Property Trustee, as the case may be, pursuant to Section 7.10 of the 
Declaration, the Company shall pay to the Delaware Trustee or the Property 
Trustee, as the case may be, all amounts accrued to the date of such 
termination, removal or resignation.

                                       10



                                      ARTICLE VI
                                    SUBORDINATION

SECTION 6.1.   SUBORDINATION.

          The indebtedness evidenced by the Debenture shall be, to the extent 
and in the manner set forth in the Indenture, subordinate and junior in right 
of payment to the prior payment in full of all Senior Indebtedness (as 
defined in the Indenture) with respect to the Debentures, and the Debentures 
shall rank PARI PASSU in right of payment with each other series of 
Securities issued under the Indenture, with the exception of any series of 
Securities which by its terms provides otherwise.

                                     ARTICLE VII
                             COVENANT TO LIST ON EXCHANGE

SECTION 7.1.   LISTING ON AN EXCHANGE.

          If the Debentures are to be distributed to the holders of Preferred 
Securities as described in Section 2.4(a), the Company will, if the 
Debentures are not already so listed, use its best efforts to list such 
Debentures on the New York Stock Exchange, Inc. or on such other exchange as 
the Preferred Securities are then listed.

                                     ARTICLE VIII
                                  FORM OF DEBENTURE

SECTION 8.1.   FORM OF DEBENTURE.

          The Debentures and the Trustee's Certificate of Authentication to 
be endorsed thereon are to be substantially in the following forms:

                             (FORM OF FACE OF DEBENTURE)

          [IF THE DEBENTURE IS TO BE A GLOBAL DEBENTURE, INSERT -- This 
Debenture is a Global Debenture within the meaning of the Indenture 
hereinafter referred to and is registered in the name of a Depository or a 
nominee of a Depository. This Debenture is exchangeable for Debentures 
registered in the name of a person other than the Depository or its nominee 
only in the limited circumstances described in the Indenture, and no transfer 
of this Debenture (other than a transfer of this Debenture as a whole by the 
Depository to a nominee of the Depository or by a nominee of the Depository 
to the Depository or another nominee of the Depository or by the Depository 
or any such nominee to a successor Depository or a nominee of such successor 
Depository) may be registered except in limited circumstances.

                                       11



          Unless this Debenture is presented by an authorized representative 
of The Depository Trust Company (55 Water Street, New York, New York) to the 
issuer or its agent for registration of transfer, exchange or payment, and 
any Debenture issued is registered in the name of Cede & Co. or such other 
name as requested by an authorized representative of The Depository Trust 
Company and any payment hereon is made to Cede & Co., ANY TRANSFER, PLEDGE OR 
OTHER USE HEREOF FOR VALUE  OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL 
since the registered owner hereof, Cede & Co., has an interest herein.]

No. ____________________
$_______________________
CUSIP No. ______________

                                       12



                          PUBLIC SERVICE COMPANY OF COLORADO

                   ___% DEFERRABLE INTEREST SUBORDINATED DEBENTURE
                                     DUE _______

          PUBLIC SERVICE COMPANY OF COLORADO, a Colorado corporation (the 
"Company", which term includes any successor corporation under the Indenture 
hereinafter referred to), for value received, hereby promises to pay to 
______________ or registered assigns, the principal sum of _____________ 
Dollars ($___________) on _________, ____, and to pay interest on said 
principal sum from ____________, 199__, or from the most recent interest 
payment date (each such date, an "Interest Payment Date") to which interest 
has been paid or duly provided for, quarterly (subject to deferral as set 
forth herein) in arrears on March 31, June 30, September 30 and December 31 
of each year commencing June 30, 1998, at the rate of ___% per annum until 
the principal hereof shall have become due and payable, and on any overdue 
principal and premium, if any, and (without duplication and to the extent 
that payment of such interest is enforceable under applicable law) on any 
overdue installment of interest at the same rate per annum compounded 
quarterly.  The amount of interest payable on any Interest Payment Date shall 
be computed on the basis of a 360-day year of twelve 30-day months.  In the 
event that any date on which interest is payable on this Debenture is not a 
Business Day, then payment of interest payable on such date will be made on 
the next succeeding day that is a Business Day (and without any interest or 
other payment in respect of any such delay), except that, if such Business 
Day is in the next succeeding calendar year, such payment shall be made on 
the immediately preceding Business Day, in each case with the same force and 
effect as if made on such date.  The interest installment so payable, and 
punctually paid or duly provided for, on any Interest Payment Date will, as 
provided in the Indenture, be paid to the person in whose name this Debenture 
(or one or more Predecessor Securities, as defined in said Indenture) is 
registered at the close of business on the regular record date for such 
interest installment, which shall be the close of business on the Business 
Day next preceding such Interest Payment Date. [IF PURSUANT TO THE PROVISIONS 
OF THE INDENTURE THE DEBENTURES ARE NO LONGER REPRESENTED BY A GLOBAL 
DEBENTURE --which shall be the close of business on the ____ day next preceding 
such Interest Payment Date.]  Any such interest installment not punctually paid 
or duly provided for shall forthwith cease to be payable to the registered 
Holders on such regular record date and may be paid to the Person in whose 
name this Debenture (or one or more Predecessor Securities) is registered at 
the close of business on a special record date to be fixed by the Trustee for 
the payment of such defaulted interest, notice whereof shall be given to the 
registered Holders of this series of Debentures not less than 10 days prior 
to such special record date, or may be paid at any time in any other lawful 
manner not inconsistent with the requirements of any securities exchange on 
which the Debentures may be listed, and upon such notice as may be required 
by such exchange, all as more fully provided in the Indenture.  The principal 
of (and premium, if any) and the interest on this Debenture shall be payable 
at the office or agency of the Trustee maintained for that purpose in any 
coin or currency of the United States of America that at the time of payment 
is legal tender for payment of public and private debts; provided, however, 
that payment of interest may be made at the option of the Company by check 
mailed to the registered Holder at such address as shall appear in the 
Security Register. 


                                       13



Notwithstanding the foregoing, so long as the Holder of this Debenture is the 
Property Trustee, the payment of the principal of (and premium, if any) and 
interest on this Debenture will be made at such place and to such account as 
may be designated by the Property Trustee.

          The indebtedness evidenced by this Debenture is, to the extent 
provided in the Indenture, subordinate and junior in right of payment to the 
prior payment in full of all Senior Indebtedness, and this Debenture is 
issued subject to the provisions of the Indenture with respect thereto.  Each 
Holder of this Debenture, by accepting the same, (a) agrees to and shall be 
bound by such provisions, (b) authorizes and directs the Trustee on his or 
her behalf to take such action as may be necessary or appropriate to 
acknowledge or effectuate the subordination so provided and (c) appoints the 
Trustee his or her attorney-in-fact for any and all such purposes.  Each 
Holder hereof, by his or her acceptance hereof, hereby waives all notice of 
the acceptance of the subordination provisions contained herein and in the 
Indenture by each holder of Senior Indebtedness, whether now outstanding or 
hereafter incurred, and waives reliance by each such holder upon said 
provisions.

          This Debenture shall not be entitled to any benefit under the 
Indenture hereinafter referred to, be valid or become obligatory for any 
purpose until the Certificate of Authentication hereon shall have been signed 
by or on behalf of the Trustee.

          The provisions of this Debenture are continued on the reverse side 
hereof and such continued provisions shall for all purposes have the same 
effect as though fully set forth at this place.

                                       14



          IN WITNESS WHEREOF, the Company has caused this instrument to be 
executed.

Dated 
      ---------------------------------

                                       PUBLIC SERVICE COMPANY OF COLORADO

                                       By:
                                          -------------------------------------
                                          Name:
                                          Title:

Attest:

By: 
- ---------------------------------------
   Name:
   Title:


                       (FORM OF CERTIFICATE OF AUTHENTICATION)

                            CERTIFICATE OF AUTHENTICATION

          This is one of the Debentures of the series of Debentures described 
in the within-mentioned Indenture.

Dated 
- ---------------------------------------

                                       THE BANK OF NEW YORK, as Trustee

                                       By:
                                          -------------------------------------
                                          Authorized Signatory

                                       15



                            (FORM OF REVERSE OF DEBENTURE)

          This Debenture is one of a duly authorized series of Securities of 
the Company (herein sometimes referred to as the "Securities"), specified in 
the Indenture, all issued or to be issued in one or more series under and 
pursuant to an Indenture dated as of ________________, 1998, duly executed 
and delivered between the Company and The Bank of New York, as Trustee (the 
"Trustee"),  as supplemented by the First Supplemented Indenture dated as of 
_______, 1998, between the Company and the Trustee (the Indenture as so 
supplemented, the "Indenture"), to which Indenture and all indentures 
supplemental thereto reference is hereby made for a description of the 
rights, limitations of rights, obligations, duties and immunities thereunder 
of the Trustee, the Company and the Holders of the Securities.  By the terms 
of the Indenture, the Securities are issuable in series that may vary as to 
amount, date of maturity, rate of interest and in other respects as provided 
in the Indenture.  This series of Securities is limited in aggregate 
principal amount as specified in said First Supplemental Indenture.

          Except as provided in the next paragraph, the Debentures may not be 
redeemed by the Company prior to ______________, ______.  The Company shall 
have the right to redeem this Debenture at the option of the Company, without 
premium or penalty, in whole or in part at any time and from time to time on 
or after ___________, ____   (an "Optional Redemption"), at a redemption 
price equal to 100% of the principal amount plus any accrued but unpaid 
interest, including any Compounded Interest and Additional Interest, if any, 
to the date of such redemption (the "Optional Redemption Price").  Any 
redemption pursuant to this paragraph will be made upon not less than 30 nor 
more than 60 days' notice, at the Optional Redemption Price.

          If a Special Event (as defined below) has occurred and is 
continuing then the Company shall have the right upon not less than 30 days 
nor more than 60 days notice to the Holders of the Debentures to redeem the 
Debentures, in whole or in part, for cash within 90 days following the 
occurrence of such Special Event (the "90 Day Period") at a redemption price 
equal to 100% of the principal amount to be redeemed plus any accrued and 
unpaid interest thereon, including Compounded Interest and Additional 
Interest, if any, to the date of such redemption (the "Redemption Price"); 
provided, however, that in the case of a Tax Event, if at the time there is 
available to the Company the opportunity to eliminate, within the 90 Day 
Period, the Tax Event by taking some ministerial action ("Ministerial 
Action"), such as filing a form or making an election, or pursuing some other 
similar reasonable measure which has no adverse effect on the Company, the 
Trust or the Holders of the Trust Securities issued by the Trust, the Company 
shall pursue such Ministerial Action in lieu of redemption, and, provided, 
further, that the Company shall have no right to redeem the Debentures while 
the Trust is pursuing any Ministerial Action pursuant to its obligations 
under the Declaration.  The Redemption Price shall be paid prior to 12:00 
noon, New York time, on the date of such redemption or such earlier time as 
the Company determines, provided that the Company shall deposit with the 
Trustee an amount sufficient to pay the Redemption Price by 10:00 a.m., New 
York time, on the date such Redemption Price is to be paid.

                                       16



          A "Special Event" shall mean either a Tax Event or an Investment 
Company Event. "Tax Event" shall mean that the Company shall have received an 
opinion of counsel (which may be regular counsel to the Company or an 
Affiliate, but not an employee thereof, which must be acceptable to the 
Property Trustee of the Trust) experienced in such matters to the effect 
that, as a result of any amendment to, or change (including any announced 
prospective change) in, the laws (or any regulations thereunder) of the 
United States or any political subdivision or taxing authority thereof or 
therein affecting taxation, or as a result of any official administrative 
pronouncement or judicial decision interpreting or applying such laws or 
regulations, which amendment or change is effective or such interpretation or 
pronouncement is announced on or after the date of original issuance of 
Preferred Securities, there is more than an insubstantial risk that (i) the 
Trust is subject to United States Federal income tax with respect to interest 
received on the Debentures, (ii) interest payable by the Company to the Trust 
on the Debentures will not be deductible for United States Federal income tax 
purposes, or (iii) the Trust is subject to more than a de minimis amount of 
other taxes, duties, assessments or other governmental charges.  "Investment 
Company Event" shall mean the occurrence of a change in law or regulation or 
a change in interpretation or application of law or regulation by any 
legislative body, court, governmental agency or regulatory authority (a 
"Change in 1940 Act Law") to the effect that the Trust is or will be 
considered an "Investment Company" that is required to be registered under 
the Investment Company Act of 1940, as amended, which Change in 1940 Act Law 
becomes effective  on or after the date of original issuance of the Preferred 
Securities.  

          Any redemption pursuant to the occurrence of a Special Event will 
be made upon not less than 30 days nor more than 60 days notice, at the 
Redemption Price.  If the Debentures are only partially redeemed by the 
Company pursuant to an Optional Redemption, the Debentures will be redeemed 
pro rata or by lot or by any other method utilized by the Trustee; provided 
that if, at the time of redemption, the Debentures are registered as a Global 
Debenture, the Depository shall determine the principal amount of such 
Debentures held by each Debenture Holder to be redeemed in accordance with 
its procedures.

          In the event of redemption of this Debenture in part only, a new 
Debenture or Debentures of this series for the unredeemed portion hereof will 
be issued in the name of the Holder hereof upon the cancellation hereof. 

          In case an Event of Default, as defined in the Indenture, shall 
have occurred and be continuing, the principal of all of the Debentures may 
be declared, and upon such declaration shall become, due and payable, in the 
manner, with the effect and subject to the conditions provided in the 
Indenture.

          The Indenture contains provisions permitting the Company and the 
Trustee, with the consent of the Holders of not less than a majority in 
aggregate principal amount of the Debentures of each series affected at the 
time outstanding, as defined in the Indenture, to execute supplemental 
indentures for the purpose of adding any provisions to or changing in any 
manner or eliminating any of the provisions of the Indenture or of any 
supplemental indenture or of modifying in any manner the rights of the 
Holders of the Debentures; provided, however, that no 

                                       17



such supplemental indenture shall, among other things, (i) reduce the 
principal amount thereof, or reduce the rate or extend the time of payment of 
interest thereon, or reduce any premium payable upon the redemption thereof, 
without the consent of the Holder of each Debenture so affected, or (ii) 
reduce the aforesaid percentage of Debentures, the Holders of which are 
required to consent to any such supplemental indenture, without the consent 
of the Holder of each Debenture then outstanding and affected thereby.  The 
Indenture also contains provisions permitting the Holders of a majority in 
aggregate principal amount of the Securities of any series at the time 
outstanding affected thereby, on behalf of all of the Holders of the 
Debentures of such series, to waive any Default or Event of Default with 
respect to such series, and its consequences, except a Default or Event of 
Default in the payment of the principal of or premium, if any, or interest on 
any of the Securities of such series or in respect of a provision which under 
the Indenture cannot be modified or amended without the consent of the Holder 
of each Outstanding Security of that series affected.  Any such consent or 
waiver  by the registered Holder of this Debenture (unless revoked as 
provided in the Indenture) shall be conclusive and binding upon such Holder 
and upon all future Holders and owners of this Debenture and of any Debenture 
issued in exchange herefor or in place hereof (whether by registration of 
transfer or otherwise), irrespective of  whether or not any notation of such 
consent or waiver is made upon this Debenture.

          No reference herein to the Indenture and no provision of this 
Debenture or of the Indenture shall alter or impair the obligation of the 
Company, which is absolute and unconditional, to pay the principal of and 
premium, if any, and interest on this Debenture at the time and place and at 
the rate and in the money herein prescribed.

          So long as no Event of Default with respect to the Debentures has 
occurred and is continuing, the Company shall have the right at any time 
during the term of the Debentures from time to time to extend the interest 
payment period of such Debentures for up to 20 consecutive quarters (an 
"Extended Interest Payment Period"), at the end of which period the Company 
shall pay all interest then accrued and unpaid (together with the interest 
thereon at the rate specified for the Debentures to the extent that payment 
of such interest is enforceable under applicable law).  In the event that the 
Company exercises this right, then (a) the Company shall not declare or pay 
dividends on, make distributions with respect to, or redeem, purchase or 
acquire, or make a liquidation payment with respect to, any of its capital 
stock (other than (i) as a result of a reclassification of the Company's 
capital stock or the exchange or conversion of one class or series of the 
Company's capital stock for another class or series of the Company's capital 
stock or (ii) the purchase of fractional interests in shares of the Company's 
capital stock pursuant to the conversion or exchange provisions of such 
capital stock or the security being converted or exchanged) or make any 
guarantee payments with respect to the foregoing), (b) the Company shall not 
make any payment of interest, principal or premium, if any, on or repay, 
repurchase or redeem any debt securities (including guarantees) issued by the 
Company that rank PARI PASSU with or junior to such Debentures, and (c) the 
Company shall not make any guarantee payments with respect to the foregoing 
(other than pursuant to the Guarantee and any similar guarantee issued by the 
Company on behalf of holders of preferred securities issued by an issuer 
holding Securities issued under the Indenture). Prior to the termination of 
any such Extended Interest Payment Period, the Company may further extend the 
interest payment period; provided, that such 

                                       18



Extended Interest Payment Period, together with all such previous and further 
extensions thereof, may not exceed 20 consecutive quarters or extend beyond 
the maturity date of the Debenture.  At the termination of any such Extended 
Interest Payment Period and upon the payment of all accrued and unpaid 
interest and any additional amount then due, the Company may commence a new 
Extended Interest Payment Period, subject to the above requirements.

          As provided in the Indenture and subject to certain limitations 
therein set forth, this Debenture is transferable by the registered Holder 
hereof on the Security Register of the Company, upon surrender of this 
Debenture for registration of transfer at the office or agency of the Trustee 
in the City of ___________ and State of ____________  accompanied by a 
written instrument or instruments of transfer in form satisfactory to the 
Company or the Trustee duly executed by the registered Holder hereof or his 
attorney duly authorized in writing, and thereupon one or more new Debentures 
of authorized denominations and for the same aggregate principal amount and 
series will be issued to the designated transferee or transferees. No service 
charge will be made for any such transfer, but the Company may require 
payment of a sum sufficient to cover any tax or other governmental charge 
payable in relation thereto.

          Prior to due presentment for registration of transfer of this 
Debenture, the Company, the Trustee, any paying agent and the Security 
Registrar may deem and treat the registered holder hereof as the absolute 
owner hereof (whether or not this Debenture shall be overdue and 
notwithstanding any notice of ownership or writing hereon made by anyone 
other than the Security Registrar) for the purpose of receiving payment of or 
on account of the principal hereof and premium, if any, and interest due 
hereon and for all other purposes, and neither the Company nor the Trustee 
nor any paying agent nor any Security Registrar shall be affected by any 
notice to the contrary.

          No recourse shall be had for the payment of the principal of or the 
interest on this Debenture, or for any claim based hereon, or otherwise in 
respect hereof, or based on or in respect of the Indenture, against any 
incorporator, stockholder, officer or director, past, present or future, as 
such, of the Company or of any predecessor or successor corporation, whether 
by virtue of any constitution, statute or rule of law, or by the enforcement 
of any assessment or penalty or otherwise, all such liability being, by the 
acceptance hereof and as part of the consideration for the issuance hereof, 
expressly waived and released.

          [This Global Debenture is exchangeable for Debentures in definitive
form only under certain limited circumstances set forth in the Indenture.  The
Debentures of this series so issued are issuable only in registered form without
coupons in denominations of $25 and any integral multiple thereof.]  As provided
in the Indenture and subject to certain limitations herein and therein set
forth, Debentures of this series so issued are exchangeable for a like aggregate
principal amount of Debentures of this series of a different authorized
denomination, as requested by the Holder surrendering the same.

          All terms used in this Debenture that are defined in the Indenture 
shall have the meanings assigned to them in the Indenture.

                                       19



                                      ARTICLE IX
                             ORIGINAL ISSUE OF DEBENTURES

SECTION 9.1.   ORIGINAL ISSUE OF DEBENTURES.

          Debentures in the aggregate principal amount of $___________ may, upon
execution of this First Supplemental Indenture, be executed by the Company and
delivered to the Trustee for authentication, and the Trustee shall thereupon
authenticate and deliver said Debentures to or upon the written order of the
Company, signed by its Chairman, its Vice Chairman, its President, or any Vice
President and its Treasurer or an Assistant Treasurer, without any further
action by the Company.


                                      ARTICLE X
                                    MISCELLANEOUS

SECTION 10.1.  RATIFICATION OF INDENTURE.

          The Indenture, as supplemented by this First Supplemental 
Indenture, is in all respects ratified and confirmed, and this First 
Supplemental Indenture shall be deemed part of the Indenture in the manner 
and to the extent herein and therein provided.

SECTION 10.2.  TRUSTEE NOT RESPONSIBLE FOR RECITALS.

          The recitals herein contained are made by the Company and not by 
the Trustee, and the Trustee assumes no responsibility for the correctness 
thereof. The Trustee makes no representation as to the validity or 
sufficiency of this First Supplemental Indenture.

SECTION 10.3.  GOVERNING LAW.

          This First Supplemental Indenture and each Debenture shall be 
deemed to be a contract made under the internal laws of the State of New 
York, and for all purposes shall be construed in accordance with the laws of 
said State.

SECTION 10.4.  SEPARABILITY.

          In case any one or more of the provisions contained in this First 
Supplemental Indenture or in the Debentures shall for any reason be held to 
be invalid, illegal or unenforceable in any respect, such invalidity, 
illegality or unenforceability shall not affect any other provisions of this 
First Supplemental Indenture or of the Debentures, but this First 
Supplemental Indenture and the Debentures shall be construed as if such 
invalid or illegal or unenforceable provision had never been contained herein 
or therein.


                                       20



SECTION 10.5.  COUNTERPARTS.

          This First Supplemental Indenture may be executed in any number of 
counterparts each of which shall be an original; but such counterparts shall 
together constitute but one and the same instrument.

                [THE REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK.]

                                       21



          IN WITNESS WHEREOF, the parties hereto have caused this First 
Supplemental Indenture to be duly executed, and their respective corporate 
seals to be hereunto affixed and attested, on the date or dates indicated in 
the acknowledgments and as of the day and year first above written.

                                       PUBLIC SERVICE COMPANY OF COLORADO

                                       By:
                                          -------------------------------------
                                          Name:
                                          Title:

Attest:

By:
- ---------------------------------------

                                       THE BANK OF NEW YORK
                                       as Trustee

                                       By:
                                          -------------------------------------
                                          Name:
                                          Title:

Attest:

By:
- ---------------------------------------


                                       22