FORM OF SUPPLEMENTAL INDENTURE

THIS                      SUPPLEMENTAL INDENTURE, dated as of                         , is between PUBLIC SERVICE COMPANY OF COLORADO, a Colorado corporation (hereinafter called the “Issuer” or the “Company”), having its principal office at 1225 17th Street, Denver, Colorado 80202, and THE BANK OF NEW YORK TRUST COMPANY, N.A., as Trustee (hereinafter called the “Trustee”), having its principal corporate trust office at 101 Barclay Street, New York, New York 10286.

Recitals of the Issuer

The Issuer has heretofore executed and delivered an Indenture, dated as of July 1, 1999 (the “Original Indenture”, the Original Indenture, previously supplemented and as further supplemented by this supplemental indenture being hereinafter referred to as the “Indenture”), relating to the issuance at any time or from time to time of its Securities on terms to be specified at the time of issuance. Terms used and not otherwise defined herein shall (unless the context otherwise clearly requires) have the respective meanings given to them in the Original Indenture.

The Original Indenture provides in Article Three thereof that, prior to the issuance of Securities of any series, the form of such Securities and the terms applicable to such series shall be established in, or pursuant to, the authority granted in a resolution of the Board of Directors or established in one or more indentures supplemental thereto.

The Issuer desires by this supplemental indenture, among other things, to establish the form of the Securities of a series, to be titled         % Notes due                                  of the Issuer, and to establish the terms applicable to such series, pursuant to Sections 201, 301 and 901 of the Original Indenture.  The Issuer has duly authorized the execution and delivery of this supplemental indenture.

Article Nine of the Original Indenture provides that the Issuer, when authorized by a resolution of its Board of Directors, and the Trustee may from time to time and at any time amend the Indenture without the consent of Securityholders for certain purposes enumerated in Section 901 thereof, including the purposes set forth in subsection (7) of said Section 901.

The execution and delivery of this supplemental indenture by the parties hereto are in all respects authorized by the provisions of the Indenture.

All things necessary have been done to make this supplemental indenture a valid agreement of the Issuer, in accordance with its terms.

NOW, THEREFORE, THIS              SUPPLEMENTAL INDENTURE WITNESSETH:

For and in consideration of the premises, it is mutually covenanted and agreed, as follows:

ARTICLE I

Establishment of % Notes due

Section 1.01.          The title of the series of the Securities established by this supplemental indenture shall be         % Notes due                                      of the Issuer (hereinafter called the “Series        Notes”).

Section 1.02.          The Series        Notes shall be limited to [$                            ] in aggregate principal amount.

Section 1.03.          The Series        Notes may be issued in whole or in part as one or more Global Securities and The Depository Trust Company, or a nominee thereof, shall be the Depository for such Global Security or Global Securities.  The Depository for such Global Security or Global Securities representing Series        Notes may surrender one or more Global Securities representing Series        Notes in exchange in whole or in part for individual Series        Notes on such terms as are acceptable to the Issuer and such Depository and otherwise subject to the terms of the Indenture.

Section 1.04.          The principal of the Series        Notes shall be payable on                                   .

Section 1.05.          The Series        Notes shall bear interest at the rate of       % per annum and shall accrue from                           . The Interest Payment Dates shall be                          and                            in each year, commencing                               . The Regular Record Dates in respect of such Interest Payment Dates shall be                                  and                                in each year, respectively.

Section 1.06.          The Corporate Trust Office of The Bank of New York Trust Company, N.A. shall be the place at which the principal of the Series        Notes shall be payable.  Any interest thereon shall be paid as specified in Section 307 of the Original Indenture.

Section 1.07.          [Interest Payment Deferral Provisions, if any.]

Section 1.08.          [Optional Redemption Provisions, if any.]

Section 1.09.          [Mandatory Redemption Provisions, if any.]

Section 1.10.                              The Series        Notes shall be issued in denominations of [$              ] and any integral multiple thereof.

Section 1.11.          [Original Issue Discount Provisions, if any.]

Section 1.12.          [Additional Defaults, if any.]

Section 1.13.          Sections 1301 and 1302 of the Indenture shall be applicable to the Series        Notes.

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Section 1.14.          [Currency, if other than U.S. Dollars.]

Section 1.15.          [Additional Covenants, if any.]

Section 1.16.          The Issuer hereby appoints, or confirms the appointment of, The Bank of New York Trust Company, N.A. as the initial Trustee, Securities Registrar and Paying Agent, subject to the provisions of the Indenture with respect to resignation, removal and succession, and subject, further, to the right of the Issuer to appoint additional agents (including Paying Agents).

Section 1.17.          [Exceptions to Business Day Definition, if any.]

Section 1.18.          [Any Additional Terms.]

Section 1.19.          The Series        Notes shall be substantially in the form set forth in Exhibit A hereto, and shall have such further terms as are reflected in such form, subject to changes in the form thereof made by the Issuer and acceptable to the Trustee.

ARTICLE II

Miscellaneous

Section 2.01.          The recitals contained herein shall be taken as the statements of the Issuer, and the Trustee assumes no responsibility for the correctness of the same. The Trustee makes no representation as to the validity of this supplemental indenture. The Indenture, as supplemented by this supplemental indenture, is in all respects hereby adopted, ratified and confirmed.

Section 2.02.          This supplemental indenture may be executed in any number of counterparts, and on separate counterparts, each of which shall be an original; but such counterparts shall together constitute but one and the same instrument.

Section 2.03.          If any provision of this supplemental indenture limits, qualifies or conflicts with the duties imposed by any of Sections 310 to 317, inclusive, of the Trust Indenture Act of 1939, as amended by the Trust Indenture Reform Act of 1990, through operation of Section 318(c), such imposed duties shall control.

Section 2.04.          The Article headings herein are for convenience only and shall not affect the interpretation hereof.

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IN WITNESS WHEREOF, the parties hereto have caused this Second Supplemental Indenture to be duly executed as of the          day of                                   .

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EXHIBIT A

Form of       % Note due                         

[THIS SECURITY IS A GLOBAL SECURITY REGISTERED IN THE NAME OF THE DEPOSITORY (REFERRED TO HEREIN) OR A NOMINEE THEREOF AND UNLESS AND UNTIL IT IS EXCHANGED IN WHOLE OR IN PART FOR THE INDIVIDUAL SECURITIES