THIRTY-SIXTH SUPPLEMENTAL INDENTURE

Exhibit 4.1

WESTAR ENERGY, INC.

TO

BNY MIDWEST TRUST COMPANY

as Trustee

(as Successor to

HARRIS TRUST AND SAVINGS BANK)

THIRTY-SIXTH SUPPLEMENTAL INDENTURE

Dated as of June 1, 2004

First Mortgage Bonds, Pollution Control Refunding Revenue Series Due 2033

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THIRTY-SIXTH SUPPLEMENTAL INDENTURE, dated as of the 1st day of June, Two Thousand and Four, made by and between Westar Energy, Inc., formerly The Kansas Power and Light Company, a corporation organized and existing under the laws of the State of Kansas (hereinafter called the “ Company ”), party of the first part, and BNY Midwest Trust Company, an Illinois trust company whose mailing address is 2 North LaSalle Street, Suite 1020, Chicago, IL 60602 (hereinafter called the “ Trustee ”), as Trustee (as successor to Harris Trust and Savings Bank), under the Mortgage and Deed of Trust dated July 1, 1939, hereinafter mentioned, party of the second part;

WHEREAS, the Company has heretofore executed and delivered to the Trustee its Mortgage and Deed of Trust dated July 1, 1939 (hereinafter referred to as the “ Original Indenture ”), to provide for and to secure the issue of First Mortgage Bonds of the Company, issuable in series, and to declare the terms and conditions upon which the Bonds (as defined in the Original Indenture) are to be issued thereunder; and

WHEREAS, the Company has heretofore executed and delivered to the Trustee Thirty-Five Supplemental Indentures supplemental to said Original Indenture, of which Thirty-Three provided for the issuance thereunder of series of the Company’s First Mortgage Bonds, and there is set forth below information with respect to such Supplemental Indentures as have provided for the issuance of Bonds, and the principal amount of Bonds which remain outstanding as of June 1, 2004.

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; and

WHEREAS, the Company is entitled at this time to have authenticated and delivered additional bonds in substitution for refundable Bonds, upon compliance with the provisions of Article III of the Original Indenture, as amended; and

WHEREAS, the Company desires by this Thirty-Sixth Supplemental Indenture (hereinafter referred to as this “ Supplemental Indenture ”) to supplement the Original Indenture and to provide for the creation of a new series of bonds under the Original Indenture to be designated “First Mortgage Bonds, Pollution Control Refunding Revenue Series Due 2033”(hereinafter called “Bonds

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of the 2033 Series); and the Original Indenture provides that certain terms and provisions, as determined by the Board of Directors of the Company, of the Bonds of any particular series may be expressed in and provided by the execution of an appropriate supplemental indenture; and

WHEREAS, the Company in the exercise of the powers and authority conferred upon and reserved to it under the provisions of the Original Indenture and indentures supplemental thereto, and pursuant to appropriate resolutions of its Board of Directors, has duly resolved and determined to make, execute and deliver to the Trustee a supplemental indenture in the form hereof for the purposes herein provided; and

WHEREAS, all conditions and requirements necessary to make this Thirty-Sixth Supplemental Indenture a valid, binding and legal instrument have been done, performed and fulfilled, and the execution and delivery hereof have been in all respects duly authorized;

NOW, THEREFORE, THIS INDENTURE WITNESSETH: That, in consideration of the premises and of the mutual covenants herein contained and of the sum of One Dollar duly paid by the Trustee to the Company at or before the time of the execution of these presents, and of other valuable considerations, the receipt whereof is hereby acknowledged, and in order further to secure the payment of the principal of and interest and premium, if any, on all Bonds at any time issued and outstanding under the Original Indenture as amended by all indentures supplemental thereto (hereinafter sometimes collectively called the “ Indenture ”) according to their tenor, purport and effect, and to declare certain terms and conditions upon and subject to which Bonds are to be issued and secured, the Company has executed and delivered this Supplemental Indenture, and by these presents grants, bargains, sells, warrants, aliens, releases, conveys, assigns, transfers, mortgages, pledges, sets over and ratifies and confirms unto BNY Midwest Trust Company, as Trustee, and to its successors in trust under the Indenture forever, all and singular the following described properties (in addition to all other properties heretofore specifically subjected to the lien of the Indenture and not heretofore released from the lien thereof), that is to say:

FIRST.

All and singular the rents, real estate, chattels real, easements, servitudes, and leaseholds of the Company, or which, subject to the provisions of Article XII of the Original Indenture, the Company may hereafter acquire, including, among other things, the property described in Appendix A hereto under the caption “First”, which description is hereby incorporated herein by reference and made a part hereof as if fully set forth herein, together with all improvements of any type located thereon.

Also all power houses, plants, buildings and other structures, dams, dam sites, substations, heating plants, gas works, holders and tanks, compressor stations, gasoline extraction plants, together with all and singular the electric heating, gas and mechanical appliances appurtenant thereto of every nature whatsoever, now owned by the Company or which it may hereafter acquire, including all and singular the machinery, engines, boilers, furnaces, generators, dynamos, turbines and motors, and all and every character of mechanical appliance for generating or producing electricity, steam, water, gas and other agencies for light, heat, cold or power or any other purpose whatsoever.

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SECOND.

Also all transmission and distribution systems used for the transmission and distribution of electricity, steam, water, gas and other agencies for light, heat, cold or power, or any other purpose whatever, whether underground or overhead or on the surface or otherwise of the Company, or which, subject to the provisions of Article XII of the Original Indenture, the Company may hereafter acquire, including all poles, posts, wires, cables, conduits, mains, pipes, tubes, drains, furnaces, switchboards, transformers, insulators, meters, lamps, fuses, junction boxes, water pumping stations, regulator stations, town border metering stations and other electric, steam, water and gas fixtures and apparatus.

THIRD.

Also all franchises and all permits, ordinances, easements, privileges and immunities and licenses, all rights to construct, maintain and operate overhead, surface and underground systems for the distribution and transmission of electricity, gas, water or steam for the supply to itself or others of light, heat, cold or power or any other purpose whatsoever, all rights-of-way, all waters, water rights and flowage rights and all grants and consents, now owned by the Company or, subject to the provisions of Article XII of the Original Indenture, which it may hereafter acquire.

Also all inventions, patent rights and licenses of every kind now owned by the Company or, subject to the provisions of Article XII of the Original Indenture, which it may hereafter acquire.

FOURTH.

Also, subject to the provisions of Article XII of the Original Indenture, all other property, real, personal and mixed (except as therein or herein expressly excepted) of every nature and kind and wheresoever situated now or hereafter possessed by or belonging to the Company, or to which it is now, or may at any time hereafter be, in any manner entitled at law or in equity.

FIFTH.

Also any and all property of any kind or description which may from time to time after the date of the Original Indenture by delivery or by writing of any kind be conveyed, mortgaged, pledged, assigned or transferred to the Trustee by the Company or by any person, copartnership or corporation, with the consent of the Company or otherwise, and accepted by the Trustee, to be held as part of the mortgaged property; and the Trustee is hereby authorized to accept and receive any such property and any such conveyance, mortgage, pledge, assignment and transfer, as and for additional security hereunder, and to hold and apply any and all such property subject to and in accordance with the terms and provisions upon which such conveyance, mortgage, pledge, assignment or transfer shall be made.

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SIXTH.

Together with all and singular, the tenements, hereditaments and appurtenances belonging or in any wise appertaining to the aforesaid property or any part thereof, with the reversion and reversions, remainder and remainders, tolls, rents, revenues, issues, income, products and profits thereof, and all the estate, right, title, interest and claim whatsoever, at law and in equity, which the Company now has or may hereafter acquire in and to the aforesaid property and franchises and every part and parcel thereof.

EXPRESSLY EXCEPTING AND EXCLUDING, HOWEVER, all properties of the character excepted from the lien of the Original Indenture.

TO HAVE AND TO HOLD all said properties, real, personal and mixed, mortgaged, pledged and conveyed by the Company as aforesaid, or intended so to be, unto the Trustee and its successors and assigns forever;

SUBJECT, HOWEVER, to the exceptions and reservations hereinabove referred to, to existing leases other than leases which by their terms are subordinate to the lien of the Indenture, to existing liens upon rights-of-way for transmission or distribution line purposes, as defined in Article I of the Original Indenture; and any extensions thereof, and subject to existing easements for streets, alleys, highways, rights-of-way and railroad purposes over, upon and across certain of the property herein before described and subject also to all the terms, conditions, agreements, covenants, exceptions and reservations expressed or provided in the deeds or other instruments respectively under and by virtue of which the Company acquired the properties hereinabove described and to undetermined liens and charges, if any, incidental to construction or other existing permitted liens as defined in Article I of the Original Indenture;

IN TRUST, NEVERTHELESS, upon the terms and trusts in the Original Indenture, and the indentures supplemental thereto, including this Supplemental Indenture, set forth, for the equal and proportionate benefit and security of all present and future holders of the Bonds and coupons issued and to be issued thereunder, or any of them, without preference of any of said Bonds and coupons of any particular series over the Bonds and coupons of any other series by reason of priority in the time of issue, sale or negotiation thereof, or by reason of the purpose of issue or otherwise howsoever, except as otherwise provided in Section 2 of Article IV of the Original Indenture.

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AND IT IS HEREBY COVENANTED, DECLARED AND AGREED, by and between the parties hereto for the benefit of those who shall hold the Bonds and coupons, or any of them, to the be issued under the Indenture as follows:

ARTICLE I

DESCRIPTION OF BONDS OF THE

2033 SERIES

SECTION 1. The 2033 series of Bonds to be executed, authenticated and delivered under and secured by the Original Indenture shall be designated as “First Mortgage Bonds, Pollution Control Refunding Revenue Series Due 2033” of the Company (herein called “ Bonds of the 2033 Series ”). The Bonds of the 2033 Series shall be executed, authenticated and delivered in accordance with the provisions of, and shall in all respects be subject to, all of the terms, conditions and covenants of the Original Indenture, as amended, and subject to all the terms, conditions and covenants of this Supplemental Indenture.

Bonds of the 2033 Series shall mature February 1, 2033 and shall bear from time to time interest at a rate equal to the interest rate then borne by the City of Wamego, Kansas, Pollution Control Refunding Revenue Bonds (Westar Energy, Inc. Project) Series 2004 (hereinafter referred to as “Revenue Bonds”), issued pursuant to the Indenture of Trust, dated as of June 1, 2004 (hereinafter referred to as the “Revenue Bond Indenture”), from the City of Wamego, Kansas, to Deutsche Bank Trust Company Americas, as trustee thereunder (the “Revenue Bond Trustee”) which rate shall initially be five percent (5.00%) per annum, payable at the same times as interest is payable on the Revenue Bonds, initially semi-annually on the first day of February and August in each year, commencing February 1, 2005. Every Bond of the 2033 Series shall be dated the date of authentication except that, notwithstanding the provisions of Section 6 of Article II of the Original Indenture, if any Bond of the 2033 Series shall be authenticated at any time subsequent to the record date (as hereinafter in this Section defined) for any interest payment date but prior to the day following such interest payment date, it shall be dated as of the day following such interest payment date, provided, however, if at the time of authentication of any Bond of the 2033 Series interest shall be in default on any Bonds of the 2033 Series, such Bond shall be dated as of the day following the interest payment date to which interest has previously been paid in full or made available for payment in full on outstanding Bonds of the 2033 Series, as the case may be, or, if no interest has been paid or made available for payment, as of the date of initial authentication and delivery of such Bond. Every Bond of the 2033 Series shall bear interest from the February 1, or August 1, next preceding the date thereof, unless such Bond shall be dated prior to February 1, 2005, in which case it shall bear interest from June 1, 2004; provided, that if interest is no longer payable on February 1 and August 1 then from the interest payment date next preceding the date thereof.

The person in whose name any Bond of the 2033 Series is registered at the close of business on any record date with regard to any interest payment date shall be entitled to receive the interest payable thereon on such interest payment date notwithstanding the cancellation of such Bond upon the transfer or exchange thereof subsequent to such record date and prior to the day following such interest payment date, unless the Company shall default in the payment of the interest due on such interest payment date, in which case such defaulted interest shall be paid to the person in whose name such Bond is registered on the date of payment of such defaulted interest. The term “ record date ” as used in this Section shall mean the close of business on the record date for the Revenue Bonds pursuant

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to the Revenue Bond Indenture. The Bonds of the 2033 Series shall be payable as to principal, premium, if any, and interest, in any coin or currency of the United States of America which at the time of payment is legal tender for public and private debts, at the agency of the Company in the City of Chicago, Illinois, or at the option of the holder thereof at the agency of the Company in the Borough of Manhattan, The City of New York, provided that at the option of the Company interest may be paid by check mailed to the holder at such holder’s registered address.

The obligation of the Company to make payments with respect to the principal of and premium, if any, and interest on Bonds of the 2033 Series shall be fully or partially, as the case may be, satisfied and discharged (1) to the extent that the then due payments with respect to the principal and premium, if any, and interest on the Revenue Bonds (other than Revenue Bonds which have been redeemed or called for redemption, or which have been surrendered in lieu of, in substitution for or in exchange for other Revenue Bonds or surrendered upon registration of transfer of Revenue Bonds to the Revenue Bond Indenture) shall have been fully or partially satisfied and discharged, and (2) to the extent that at the time any payment of principal or premium, if any, or interest on the Revenue Bond becomes due, either at or before maturity, there shall be in the Bond Fund established pursuant to the Revenue Bond Indenture sufficient cash (whether received by the Revenue Bond Trustee under the Sublease dated as of June 1, 2004, (the “Sublease”), between the City of Wamego, Kansas, and the Company, or otherwise) available to pay the principal of and premium, if any, and interest on the Revenue Bonds then due. The obligation of the Company to make payments with respect to the principal of and premium, if any, and interest on the Bonds of the 2033 Series shall also be satisfied and discharged at such time pursuant to the provisions of Article X of the Revenue Bond Indenture, the principal and premium, if any, and interest on the Revenue Bonds has been paid or the Revenue Bond Trustee shall hold moneys or Investment Obligations (meeting the requirements of such Article X) sufficient to pay the principal of and premium, if any, and interest on the Revenue Bonds and to the extent in such Article X provided, and provision for paying all other sums required by such Article X shall have been made. In this case, the entire principal amount of the Bonds of the 2033 Series then outstanding shall be deemed to be satisfied and discharged. Neither the invalidity nor the unenforceability, in whole or in part, of the Sublease or any provision thereof (whether by reason of bankruptcy or otherwise) shall impair or limit the obligation of the Company to pay the principal of and premium, if any, and interest on the Bonds of the 2033 Series when and as they become due and payable. The Trustee may conclusively presume that the obligation of the Company to make payment with respect to the principal of and premium, if any, and interest on the Bonds of the 2033 Series shall have been satisfied and discharged unless and until the Trustee shall have received a written notice from the Revenue Bond Trustee stating (i) that timely payment of the principal of or premium, if any, or interest on the Revenue Bonds has not been made, (ii) that there are not sufficient available funds in such Bond Fund to make such payment and (iii)the amount of funds required to make such payment.

SECTION 2. The Bonds of the 2033 Series shall be registered bonds without coupons of the denominations of $5,000 and of any multiples of $5,000, numbered consecutively from R 1 upwards. Bonds of the 2033 Series may each be interchanged for each other bonds within a respective Series in authorized denominations and in the same aggregate principal amounts, without charge, except for any tax or governmental charge imposed in connection with such interchange.

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SECTION 3. The Bonds of the 2033 Series, and the Trustee’s Certificate with respect thereto, shall be substantially in the following forms, respectively:

[FORM OF BOND OF THE 2033 SERIES]

CUSIP             

WESTAR ENERGY, INC.

(Incorporated under the laws of the State of Kansas)

FIRST MORTGAGE BOND, POLLUTION CONTROL REFUNDING REVENUE SERIES DUE 2033

DUE February 1, 2033

WESTAR ENERGY, INC., a corporation organized and existing under the laws of the State of Kansas (hereinafter called the “ Company ”, which term shall include any successor entity as defined in the Indenture hereinafter referred to), for value received, hereby promises to pay to              or registered assigns, on the 1st day of February, 2033, the sum of              Dollars in any coin or currency of the United States of America which at the time of payment is legal tender for public and private debts, and to pay interest thereon in like coin or currency from the first day of February or August next preceding the date of this Bond (or the interest payment date next preceding the date hereof, if different from February 1 or August 1), unless no interest has been paid on this Bond, in which case from June 1, 2004, at a rate equal to the interest rate then borne by the City of Wamego, Kansas, Pollution Control Refunding Revenue Bonds (Westar Energy, Inc. Project) Series 2004 (hereinafter referred to as “ Revenue Bonds ”), issued pursuant to the Indenture of Trust, dated as of June, 1 2004 (hereinafter referred to as the “ Revenue Bond Indenture ”), from the City of Wamego, Kansas, to Deutsche Bank Trust Company Americas, as trustee thereunder (the “ Revenue Bond Trustee ”) which rate shall initially be five percent (5.00%) per annum, payable semiannually, on the first days of February and August in each year, commencing February 1, 2005, or as otherwise provided in the Indenture, until maturity, or, if this Bond shall be duly called for redemption or submitted for repurchase, until the redemption date or repurchase date, as the case may be, or, if the Company shall default in the payment of the principal or premium hereof, until the Company’s obligation with respect to the payment of such principal or premium shall be discharged as provided in the Indenture hereinafter mentioned. The interest payable on any interest payment date as aforesaid will be paid to the person in whose name this Bond is registered at the close of business on the record date for the Revenue Bonds pursuant to the Revenue Bond Indenture (the “ record date ”), unless the Company shall default in the payment of the interest due on such interest payment date, in which case such defaulted interest shall be paid to the person in whose name this Bond is registered on the date of payment of such defaulted interest. Principal of, premium, if any, and interest on, this Bond are payable at the agency of the Company in the City of Chicago, Illinois in immediately available funds or at the option of the holder thereof at the agency of the Company in the Borough of Manhattan, The City of New York, provided that at the option of the Company interest may be paid by check mailed to the holder at such holder’s registered address.

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The obligation of the Company to make payments with respect to the principal of and premium, if any, and interest on Bonds of this Series shall be satisfied and discharged (1) to the extent that the then due payments with respect to the principal and premium, if any, and interest on the Revenue Bonds (other than Revenue Bonds which have been redeemed or called for redemption, or which have been surrendered in lieu of, in substitution for or in exchange for other Revenue Bonds or surrendered upon registration of transfer of Revenue Bonds to the Revenue Bond Indenture), shall have been fully or partially satisfied and discharged, and (2) to the extent that at the time any payment of principal or premium, if any, or interest on the Revenue Bond becomes due, either at or before maturity, there shall be in the Bond Fund established pursuant to the Revenue Bond Indenture sufficient cash (whether received by the Revenue Bond Trustee under the Sublease dated as of June 1, 2004, (the “Sublease”), between the City of Wamego, Kansas, and the Company, or otherwise) available to pay the principal of and premium, if any, and interest on the Revenue Bonds then due, and (3) at such time pursuant to the provisions of Article X of the Revenue Bond Indenture, the principal and premium, if any, and interest on the Revenue Bonds has been paid or the Revenue Bond Trustee shall hold moneys or Investment Obligations (meeting the requirements of such Article X) sufficient to pay the principal of and premium, if any, and interest on the Revenue Bonds and to the extent in such Article X provided, and provision for paying all other sums required by such Article X shall have been made.

This Bond is one of a duly authorized issue of Bonds of the Company (herein called the “ Bonds ”), in unlimited aggregate principal amount, of the series hereinafter specified, all issued and to be issued under and equally secured by a Mortgage and Deed of Trust, dated July 1, 1939, executed by the Company to BNY Midwest Trust Company (herein called the “ Trustee ”), as Trustee (as successor to Harris Trust and Savings Bank), as amended by the indentures supplemental thereto including the thirty-sixth indenture supplemental thereto dated as of June 1, 2004 (herein called the “ Supplemental Indenture ”), between the Company and the Trustee (said Mortgage and Deed of Trust, as so amended, being herein called the “ Indenture ”), to which Indenture and all indentures supplemental thereto reference is hereby made for a description of the properties mortgaged and pledged, the nature and extent of the security, the rights of the bearers or registered owners of the Bonds and of the Trustee in respect thereto, and the terms and conditions upon which the Bonds are, and are to be, secured. The Bonds may be issued in series, for various principal sums, may mature at different times, may bear interest at different rates and may otherwise vary as in the Indenture provided. This Bond is one of a series designated as the “First Mortgage Bonds, Pollution Control Refunding Revenue Series Due 2033” (herein called “ Bonds of the 2033 Series ”) of the Company, issued under and secured by the Indenture executed by the Company to the Trustee.

To the extent permitted by, and as provided in the Indenture, modifications or alterations of the Indenture or of any indenture supplemental thereto, and of the rights and obligations of the Company and of the holders of the Bonds and coupons, may be made with the consent of the Company by an affirmative vote of not less than 60% in principal amount of the Bonds entitled to vote then outstanding, at a meeting of Bondholders called and held as provided in the Indenture, and

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by an affirmative vote of not less than 60% in principal amount of the Bonds of any series entitled to vote then outstanding and affected by such modification or alteration, in case one or more but less than all of the series of Bonds then outstanding under the Indenture are so affected. No modification or alteration shall be made which will affect the terms of payment of the principal of or premium, if any, or interest on, this Bond, which are unconditional. The Company has reserved the right to make certain amendments to the Indenture, without any consent or other action by holders of the Bonds of this series (i) to the extent necessary from time to time to qualify the Indenture under the Trust Indenture Act of 1939, (ii) to delete the requirement that the Company meet a net earnings test as a condition to authenticating additional Bonds or merging into another company and (iii) to make certain other amendments which make the provisions for the release of mortgaged property less restrictive, all as more fully provided in the Indenture and in the Supplemental Indenture. In addition, once all Bonds issued prior to January 1, 1997 are no longer outstanding, the Company will be permitted to issue additional Bonds in an amount equal to 70% of the value of net bondable property additions not subject to an unfunded prior lien, as provided in the Original Indenture.

The Bonds of the 2033 Series are subject to immediate redemption in whole, at the principal amount thereof, but without premium, together with accrued interest to the redemption date, upon receipt by the Trustee of a written demand from the Revenue Bond Trustee stating that the principal amount of all the Revenue Bonds then outstanding under the Revenue Bond Indenture has been declared immediately due and payable pursuant to Section 7.2 of the Revenue