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                                                                    EXHIBIT 10.5

                                 AMENDMENT NO. 4
                         Decommissioning Trust Agreement
                                 (PVNGS Unit 3)

         This Amendment No. 4 dated as of December 19, 2003, to the
Decommissioning Trust Agreement (PVNGS Unit 3), dated as of July 1, 1991, as
amended by Amendment No. 1 thereto dated as of December 1, 1994, Amendment No. 2
thereto dated as of December 16, 1996, and Amendment No. 3 thereto dated as of
March 18, 2002 (the "Decommissioning Trust Agreement", terms used herein as
therein defined), is entered into between Arizona Public Service Company ("APS")
and Mellon Bank, N.A., as Decommissioning Trustee ("Decommissioning Trustee").

                                R E C I T A L S:

         WHEREAS, the parties hereto wish to amend the Decommissioning Trust
Agreement.

         NOW, THEREFORE, in consideration of the premises and of other good and
valuable consideration, receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

         SECTION 1. Amendment.

                  (a)      A new Section 29 will be added at the end of the
                           Decommissioning Trust Agreement and will read in full
                           as follows:

                           Section 29. Notice Regarding Disbursements or
                           Payments. Notwithstanding anything to the contrary in
                           this Agreement, except for (i) payments of ordinary
                           administrative costs (including taxes) and other
                           incidental expenses of the Funds (including legal,
                           accounting, actuarial, and trustee expenses) in
                           connection with the operation of the Funds, (ii)
                           withdrawals being made under 10 CFR 50.82(a)(8), and
                           (iii) transfers between the Funds in accordance with
                           the provisions of this Agreement, no disbursement or
                           payment may be made from the Funds until written
                           notice of the intention to make a disbursement or
                           payment has been given to the Director, Office of
                           Nuclear Reactor Regulation, or the Director, Office
                           of Nuclear Material Safety and Safeguards, as
                           applicable, at least 30 working days before the date
                           of the intended disbursement or payment. The
                           disbursement or payment from the Funds, if it is
                           otherwise in compliance with the terms and conditions
                           of this Agreement, may be made following the
                           30-working day notice period if no written notice of
                           objection from the Director, Office of Nuclear
                           Reactor Regulation, or the Director, Office of
                           Nuclear Material Safety and Safeguards, as
                           applicable, is received by the Decommissioning
                           Trustee or APS within the notice period. The required
                           notice may be made by the Decommissioning Trustee or
                           on the Decommissioning Trustee's behalf. No such
                           notice is required for withdrawals being made
                           pursuant to 10 CFR 50.82(a)(8)(ii), including
                           withdrawals made during the operating life of Unit 3
                           to be used

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                           for decommissioning planning. In addition, no such
                           notice is required to be made to the NRC after
                           decommissioning has begun and withdrawals are being
                           made under 10 CFR 50.82(a)(8). This Section 29 is
                           intended to qualify each and every provision of this
                           Agreement allowing distributions from the Funds,
                           including but not limited to Section 10 hereof, and
                           in the event of any conflict between any such
                           provision and this Section 29, the provisions of this
                           Section 29 shall control.

         SECTION 2. Miscellaneous

                  (a)      Full Force and Effect.

         Except as expressly provided herein, the Decommissioning Trust
Agreement shall remain unchanged and in full force and effect. Each reference in
the Decommissioning Trust Agreement and in any exhibit or schedule thereto to
"this Agreement," "hereto," "hereof" and terms of similar import shall be deemed
to refer to the Decommissioning Trust Agreement as amended hereby.

                  (b)      Counterparts/Representations.

         The Amendment No. 4 may be executed in any number of counterparts, all
of which taken together shall constitute one and the same instrument, and any of
the parties hereto may execute this Amendment No. 4 by signing any such
counterpart. Each party represents and warrants to the other that it has full
authority to enter into this Amendment upon the terms and conditions hereof and
that the individual executing this Amendment on its behalf has the requisite
authority to bind that Party.

         IN WITNESS WHEREOF, the parties hereto have caused this Amendment No. 4
to the Decommissioning Trust Agreement to be duly executed as of the day and
year first above written.

                                        ARIZONA PUBLIC SERVICE COMPANY

                                        By: Barbara M. Gomez
                                           -------------------------------------
                                        Title: Treasurer
                                              ----------------------------------
                                        MELLON BANK, N.A. as Decommissioning
                                        Trustee

                                        By: Carlos Pacheco
                                           -------------------------------------
                                        Title: Vice President
                                              ----------------------------------

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STATE OF ARIZONA                    )
                                    ) ss:
County of Maricopa                  )

         The foregoing instrument was acknowledged before me this 12th day of
December, 2003, by Barbara M. Gomez, the Treasurer of ARIZONA PUBLIC SERVICE
COMPANY, an Arizona corporation, on behalf of said corporation.

                                        Debra L. Blondin
                                        ----------------------------------------

                                        Notary Public

My commission expires:

June 7, 2004
--------------------------

COMMONWEALTH OF PENNSYLVANIA                              )
                                                          ) ss:
County of Allegheny                                       )

         The foregoing instrument was acknowledged before me this 19th day of
December, 2003, by Carlos Pacheco, a Vice President of Mellon Bank, N.A. a
national banking association having trust powers, as Decommissioning Trustee, on
behalf of said national banking association.

                                        Julie Ann Mosco
                                        ----------------------------------------

                                        Notary Public

My commission expires:

October 13, 2007
----------------------------

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