SUPPLEMENTAL INDENTURE Dated as of April 23,2008 COMMONWEALTH EDISON COMPANY to BNY MIDWEST TRUST COMPANY and D.G. DONOVAN Trustees Under Mortgage Dated July 1, 1923, and Certain Indentures Supplemental Thereto Providing for Issuance of FIRST MORTGAGE BONDS, POLLUTION CONTROL SERIES 2008D Due March 1, 2020 and FIRST MORTGAGE BONDS, POLLUTION CONTROL SERIES 2008F Due March 1, 2017

Exhibit 4.1

This instrument was prepared by,

and when recorded should be

returned to:

Richard W. Astle

Sidley Austin LLP

One South Dearborn Street – Suite 3000

Chicago, Illinois 60603

SUPPLEMENTAL INDENTURE

Dated as of April 23, 2008

COMMONWEALTH EDISON COMPANY

to

BNY MIDWEST TRUST COMPANY

and

D.G. DONOVAN

Trustees Under Mortgage Dated July 1, 1923,

and Certain

Indentures Supplemental Thereto

Providing for Issuance of

FIRST MORTGAGE BONDS, POLLUTION CONTROL SERIES 2008D

Due March 1, 2020

and

FIRST MORTGAGE BONDS, POLLUTION CONTROL SERIES 2008F

Due March 1, 2017

THIS SUPPLEMENTAL INDENTURE, dated as of April 23, 2008, between COMMONWEALTH EDISON COMPANY, a corporation organized and existing under the laws of the State of Illinois (hereinafter called the “Company”) having an address at 440 South LaSalle Street, Suite 3300, Chicago, Illinois 60605, party of the first part, BNY MIDWEST TRUST COMPANY, a trust company organized and existing under the laws of the State of Illinois having an address at 2 North LaSalle Street, Suite 1020, Chicago, Illinois 60602, and D.G. DONOVAN, an individual having an address at 2 North LaSalle Street, Suite 1020, Chicago, Illinois 60602, as Trustee and Co-Trustee, respectively, under the Mortgage of the Company dated July 1, 1923, as amended and supplemented by Supplemental Indenture dated August 1, 1944 and the subsequent supplemental indentures hereinafter mentioned, parties of the second part (said Trustee being hereinafter called the “Trustee”, the Trustee and said Co-Trustee being hereinafter together called the “Trustees”, and said Mortgage dated July 1, 1923, as amended and supplemented by said Supplemental Indenture dated August 1, 1944 and subsequent supplemental indentures, being hereinafter called the “Mortgage”),

W I T N E S S E T H:

WHEREAS, the Company duly executed and delivered the Mortgage to provide for the issue of, and to secure, its bonds, issuable in series and without limit as to principal amount except as provided in the Mortgage; and

WHEREAS, the Company from time to time has executed and delivered supplemental indentures to the Mortgage to provide for (i) the creation of additional series of bonds secured by the Mortgage, (ii) the amendment of certain of the terms and provisions of the Mortgage and (iii) the confirmation of the lien of the Mortgage upon property of the Company, such supplemental indentures that are currently effective and the respective dates, parties thereto and purposes thereof, being as follows:

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WHEREAS, the respective designations, maturity dates and principal amounts of the bonds of each series presently outstanding under, and secured by, the Mortgage and the several supplemental indentures above referred to, are as follows:

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WHEREAS, the Mortgage provides for the issuance from time to time thereunder, in series, of bonds of the Company for the purposes and subject to the limitations therein specified; and

WHEREAS, the Company desires, by this Supplemental Indenture, to create additional series of bonds to be issuable under the Mortgage, such bonds to be designated “First Mortgage Bonds, Pollution Control Series 2008D” (hereinafter called the “bonds of Series 2008D”) and “First Mortgage Bonds, Pollution Control Series 2008F” (hereinafter called the “bonds of Series 2008F”), and the terms and provisions to be contained in the bonds of Series 2008D and the bonds of Series 2008F or to be otherwise applicable thereto to be as set forth in this Supplemental Indenture; and

WHEREAS, the bonds of Series 2008D and the Trustee’s certificate to be endorsed thereon shall be substantially in the forms included in Exhibit A hereto, and the bonds of Series 2008F and the Trustee’s certificate to be endorsed thereon shall be substantially in the forms included in Exhibit B hereto; and

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WHEREAS, the Company is legally empowered and has been duly authorized by the necessary corporate action and by order of the Illinois Commerce Commission to make, execute and deliver this Supplemental Indenture, and to create, as additional series of bonds of the Company, the bonds of Series 2008D and the bonds of Series 2008F, and all acts and things whatsoever necessary to make this Supplemental Indenture, when executed and delivered by the Company and the Trustees, a valid, binding and legal instrument, and to make the bonds of Series 2008D and the bonds of Series 2008F, when authenticated by the Trustee and issued as provided in the Mortgage and in this Supplemental Indenture, the valid, binding and legal obligations of the Company, entitled in all respects to the security of the Mortgage, as amended and supplemented, have been done and performed;

NOW, THEREFORE, in consideration of the premises and of the sum of one dollar duly paid by the Trustees to the Company, and for other good and valuable consideration, the receipt of which is hereby acknowledged, the parties hereto do hereby agree as follows:

ARTICLE I

DEFINITIONS AND RULES OF CONSTRUCTION

SECTION 1.01. Terms of the Mortgage. The terms used in this Supplemental Indenture which are defined in the Mortgage, unless otherwise specified herein, are used herein with the same meanings as in the Mortgage.

SECTION 1.02. Definitions of New Terms. The following terms shall have the following meanings in this Supplemental Indenture:

“IFA” shall mean the Illinois Finance Authority, a political subdivision and body politic and corporate duly organized and validly existing under and by virtue of the laws of the State of Illinois.

“IFA 2008D Bonds” shall mean those certain Pollution Control Revenue Refunding Bonds (Commonwealth Edison Company Project) Series 2008D issued in the original aggregate principal amount of $50,000,000 under and pursuant to the terms of the IFA 2008D Indenture.

“IFA 2008D Indenture” shall mean that certain Bond Indenture dated as of April 15, 2008, between IFA, as issuer, and The Bank of New York Trust Company, N.A., as trustee, relating to the IFA 2008D Bonds.

“IFA 2008F Bonds” shall mean those certain Pollution Control Revenue Refunding Bonds (Commonwealth Edison Company Project) Series 2008F issued in the original aggregate principal amount of $91,000,000 under and pursuant to the terms of the IFA 2008F Indenture.

“IFA 2008F Indenture” shall mean that certain Bond Indenture dated as of April 15, 2008, between IFA, as issuer, and The Bank of New York Trust Company, N.A., as trustee, relating to the IFA 2008F Bonds.

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SECTION 1.03. Rules of Construction. All references to any agreement refer to such agreement as modified, varied, or amended from time to time by the parties thereto (including any permitted successors or assigns) in accordance with its terms.

ARTICLE II

SECTION 2.01. Designation and Issuance of Bonds. (a) The bonds of Series 2008D shall, as hereinbefore recited, be designated as the Company’s “First Mortgage Bonds, Pollution Control Series 2008D.” Subject to the provisions of the Mortgage, the bonds of Series 2008D shall be issuable without limitation as to the aggregate principal amount thereof.

(b) The bonds of Series 2008F shall, as hereinbefore recited, be designated as the Company’s “First Mortgage Bonds, Pollution Control Series 2008F.” Subject to the provisions of the Mortgage, the bonds of Series 2008F shall be issuable without limitation as to the aggregate principal amount thereof.

SECTION 2.02. Forms, Dates, Maturity Dates, Interest Rates and Interest Payment Dates of Bonds. (a) (i) The definitive bonds of Series 2008D shall be in engraved, lithographed, printed or type-written form and shall be registered bonds without coupons, and such bonds and the Trustee’s certificate to be endorsed thereon shall be substantially in the forms included in Exhibit A hereto. The bonds of Series 2008D shall be dated as provided in Section 3.01 of the Mortgage, as amended by Supplemental Indenture dated April 1, 1967. All bonds of Series 2008D shall mature on March 1, 2020.

(ii) The bonds of Series 2008D shall bear interest on each day that they are outstanding at a rate per annum which is equal to the weighted-average interest rate borne on the IFA 2008D Bonds outstanding on such date; provided, however, such interest rate on the bonds of Series 2008D shall not exceed 12% per annum. The bonds of Series 2008D shall bear interest until the principal thereof shall be paid in full. Interest on the bonds of Series 2008D shall be payable to the record holder thereof on the dates that interest is payable on the IFA 2008D Bonds.

(iii) The interest on the bonds of Series 2008D so payable on any interest payment date shall, subject to the exceptions provided in Section 3.01 of the Mortgage, as amended by said Supplemental Indenture dated April 1, 1967, be paid to the person in whose name such bond is registered on such interest payment date.

(b) (i) The definitive bonds of Series 2008F shall be in engraved, lithographed, printed or type-written form and shall be registered bonds without coupons, and such bonds and the Trustee’s certificate to be endorsed thereon shall be substantially in the forms included in Exhibit B hereto. The bonds of Series 2008F shall be dated as provided in Section 3.01 of the Mortgage, as amended by Supplemental Indenture dated April 1, 1967. All bonds of Series 2008F shall mature on March 1, 2017.

(ii) The bonds of Series 2008F shall bear interest on each day that they are outstanding at a rate per annum which is equal to the weighted-average interest rate borne on the IFA 2008F Bonds outstanding on such date; provided, however, such interest rate

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on the bonds of Series 2008F shall not exceed 12% per annum. The bonds of Series 2008F shall bear interest until the principal thereof shall be paid in full. Interest on the bonds of Series 2008F shall be payable to the record holder thereof on the dates that interest is payable on the IFA 2008F Bonds.

(iii) The interest on the bonds of Series 2008F so payable on any interest payment date shall, subject to the exceptions provided in Section 3.01 of the Mortgage, as amended by said Supplemental Indenture dated April 1, 1967, be paid to the person in whose name such bond is registered on such interest payment date.

SECTION 2.03. Bonds Issued as Collateral Security. (a) The bonds of Series 2008D shall be issued, delivered, and pledged to, and registered in the name of, the trustee under the IFA 2008D Indenture in order to secure and provide for, and as collateral security for, the due and punctual payment of the principal, premium, if any, and interest due from time to time on the IFA 2008D Bonds.

(b) The bonds of Series 2008F shall be issued, delivered, and pledged to, and registered in the name of, the trustee under the IFA 2008F Indenture in order to secure and provide for, and as collateral security for, the due and punctual payment of the principal, premium, if any, and interest due from time to time on the IFA 2008F Bonds.

SECTION 2.04. Credit for Payments on IFA Bonds. (a) (i) The Company shall receive a credit against its obligation to make any payment of interest on the bonds of Series 2008D, whether on an interest payment date, at maturity, upon redemption, upon acceleration or otherwise, in an amount equal to the amount, if any, paid by or for the account of the Company in respect of any corresponding payment of interest on the IFA 2008D Bonds. So long as all the bonds of Series 2008D are pledged as described in Section 2.03(a), the obligation of the Company to make any payment with respect to the principal of the bonds of Series 2008D shall be credited in full if, at the time that any such payment of principal shall be due, there shall have been paid by or for the account of the Company the then due principal of all IFA 2008D Bonds which are outstanding. Notwithstanding the foregoing, a payment on the IFA Series 2008D Bonds made by a draw on a Credit Facility (as defined in the IFA 2008D Indenture) shall not be deemed a payment by or for the account of the Company unless and until the Company reimburses the Credit Facility Provider (as defined in the IFA 2008D Indenture) for such draw, together with any accrued interest thereon, under the Credit Facility.

(ii) The Trustee may conclusively presume that the obligation of the Company to pay the principal of, and premium, if any, and interest on, the bonds of Series 2008D as the same shall become due and payable has been credited in accordance with this Section 2.04(a) unless and until it shall have received a written notice (including a telex, telegram, telecopy or other form of written telecommunication) from the trustee under the IFA 2008D Indenture stating that payment of the principal of, or premium, if any, or interest on, the IFA 2008D Bonds has become due and payable and has not been fully paid and specifying the amount of funds required to make such payment.

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(b) (i) The Company shall receive a credit against its obligation to make any payment of interest on the bonds of Series 2008F, whether on an interest payment date, at maturity, upon redemption, upon acceleration or otherwise, in an amount equal to the amount, if any, paid by or for the account of the Company in respect of any corresponding payment of interest on the IFA 2008F Bonds. So long as all the bonds of Series 2008F are pledged as described in Section 2.03(b), the obligation of the Company to make any payment with respect to the principal of the bonds of Series 2008F shall be credited in full if, at the time that any such payment of principal shall be due, there shall have been paid by or for the account of the Company the then due principal of all IFA 2008F Bonds which are outstanding. Notwithstanding the foregoing, a payment on the IFA Series 2008F Bonds made by a draw on a Credit Facility (as defined in the IFA 2008F Indenture) shall not be deemed a payment by or for the account of the Company unless and until the Company reimburses the Credit Facility Provider (as defined in the IFA 2008F Indenture) for such draw, together with any accrued interest thereon, under the Credit Facility.

(ii) The Trustee may conclusively presume that the obligation of the Company to pay the principal of, and premium, if any, and interest on, the bonds of Series 2008F as the same shall become due and payable has been credited in accordance with this Section 2.04(b) unless and until it shall have received a written notice (including a telex, telegram, telecopy or other form of written telecommunication) from the trustee under the IFA 2008F Indenture stating that payment of the principal of, or premium, if any, or interest on, the IFA 2008F Bonds has become due and payable and has not been fully paid and specifying the amount of funds required to make such payment.

SECTION 2.05. Execution of Bonds. The bonds of Series 2008D and bonds of Series 2008F shall be executed on behalf of the Company by its President or one of its Vice Presidents, manually or by facsimile signature, and shall have its corporate seal affixed thereto or a facsimile of such seal imprinted thereon, attested by its Secretary or one of its Assistant Secretaries, manually or by facsimile signature, all as may be provided by resolution of the Board of Directors of the Company. In case any officer or officers whose signature or signatures, manual or facsimile, shall appear upon any bond of Series 2008D or any bond of Series 2008F shall cease to be such officer or officers before such bond shall have been actually authenticated and delivered, such bond nevertheless may be issued, authenticated and delivered with the same force and effect as though the person or persons whose signature or signatures, manual or facsimile, appear thereon had not ceased to be such officer or officers of the Company.

SECTION 2.06. Medium and Places of Payment of Principal of, and Premium, If Any, and Interest on, Bonds; Transferability and Exchangeability. The principal of, and premium, if any, and the interest on the bonds of Series 2008D and the bonds of Series 2008F shall be payable in any coin or currency of the United States of America which at the time of payment is legal tender for the payment of public and private debts, and such principal, premium, if any, and interest shall be payable at the office or agency of the Company in the City of Chicago, State of Illinois, and such bonds shall be transferable and exchangeable, in the manner provided in Sections 3.09 and 3.10 of the Mortgage, at said office or agency. No charge shall be made by the Company to the registered owner of any bond of Series 2008D or any bond of

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Series 2008F for the registration of transfer of such bond or for the exchange thereof for bonds of the same series of other authorized denominations, except, in the case of any transfer, a charge sufficient to reimburse the Company for any stamp or other tax or governmental charge required to be paid by the Company or the Trustee.

SECTION 2.07. Denominations and Numbering of Bonds. The bonds of Series 2008D and the bonds of Series 2008F shall be issued in the denomination of $1,000 and in such multiples of $1,000 as shall from time to time hereafter be determined and authorized by the Board of Directors of the Company or by any officer or officers of the Company authorized to make such determination, the authorization of the denomination of any bond of Series 2008D or any bond of Series 2008F to be conclusively evidenced by the execution thereof on behalf of the Company. Bonds of Series 2008D shall each be numbered R-1 and consecutively upwards, and bonds of Series 2008F shall each be numbered R-1 and consecutively upwards.

SECTION 2.08. Temporary Bonds. Until definitive bonds of Series 2008D or definitive bonds of Series 2008F are ready for delivery, there may be authenticated and issued in lieu of any thereof and subject to all of the provisions, limitations, and conditions set forth in Section 3.11 of the Mortgage, temporary registered bonds of Series 2008D without coupons or temporary registered bonds of Series 2008F without coupons, respectively.

SECTION 2.09. Optional Redemption of Bonds. (a) Upon the notice and in the manner provided in Sections 4.01 and 4.03 of the IFA 2008D Indenture, the bonds of Series 2008D may be redeemed in whole or in part, at the option of the Company, on the date or dates determined under Section 4.01 of the IFA 2008D Indenture, at the redemption prices (expressed as percentages of the principal amount of each bond of Series 2008D or portion thereof to be redeemed) set forth in Section 4.01 of the IFA 2008D Indenture, plus accrued interest to the redemption date.

(b) Upon the notice and in the manner provided in Sections 4.01 and 4.03 of the IFA 2008F Indenture, the bonds of Series 2008F may be redeemed in whole or in part, at the option of the Company, on the date or dates determined under Section 4.01 of the IFA 2008F Indenture, at the redemption prices (expressed as percentages of the principal amount of each bond of Series 2008F or portion thereof to be redeemed) set forth in Section 4.01 of the IFA 2008F Indenture, plus accrued interest to the redemption date.

SECTION 2.10. Mandatory Redemption. (a) Upon the notice and in the manner provided in Section 4.11 of the IFA 2008D Indenture, the bonds of Series 2008D shall be redeemed by the Company in whole, or as provided under such paragraphs in part, at 100% of the principal amount thereof plus accrued interest to the redemption date.

(b) Upon the notice and in the manner provided in Section 4.11 of the IFA 2008F Indenture, the bonds of Series 2008F shall be redeemed by the Company in whole, or as provided under such paragraphs in part, at 100% of the principal amount thereof plus accrued interest to the redemption date.

SECTION 2.11. Default Mandatory Redemption. (a) The bonds of Series 2008D shall be redeemed promptly, without notice, by the Company in whole at 100% of the principal

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amount thereof plus accrued interest to the date of redemption following receipt by the Trustee of written notice from the trustee under the IFA 2008D Indenture stating that the principal of the IFA 2008D Bonds has been declared to be immediately due and payable as a result of an event of default under the IFA 2008D Indenture.

(b) The bonds of Series 2008F shall be redeemed promptly, without notice, by the Company in whole at 100% of the principal amount thereof plus accrued interest to the date of redemption following receipt by the Trustee of written notice from the trustee under the IFA 2008F Indenture stating that the principal of the IFA 2008F Bonds has been declared to be immediately due and payable as a result of an event of default under the IFA 2008F Indenture.

ARTICLE III

AMENDMENT OF PROVISION OF MORTGAGE

Section 15.06 of the Mortgage shall be amended and restated to read in its entirety as follows:

SECTION 15.06. The Trustee and any successor to the Trustee may resign and be discharged from the trusts created by this Mortgage by giving notice thereof in writing to the Company, specifying the date when such resignation shall take effect, and by giving notice thereof to the bondholders in the manner and to the extent provided under Section 15.10(c), and by publishing such notice at least once a week for three successive calendar weeks (the first such publication to be not less than thirty days nor more than sixty days prior to the effective date of such resignation) in one authorized newspaper in the City of Chicago, State of Illinois, and in one authorized newspaper in the Borough of Manhattan, The City of New York, State of New York. Subject to the provisions of Sections 15.04 and 15.05, such resignation shall take effect on the date specified in such notice unless previously a successor Trustee shall have been appointed as hereinafter provided, in which event such resignation shall take effect upon the appointment of such successor Trustee. The Co-Trustee and any successor to the Co-Trustee may resign at any time and be discharged from the trusts hereby created by giving the Trustee and the Company notice in writing of such resignation, specifying a date when such resignation shall take effect, which shall be at least thirty days after the giving of such notice. Such resignation shall, subject to the provisions of Sections 15.04 and 15.05, take effect on the date specified in such notice unless previously a successor trustee shall have been appointed as hereinafter provided, in which event such resignation shall take effect immediately upon the appointment of such a successor trustee.

Either of the Trustees or any successor trustee may be removed at any time by the holders of a majority in principal amount of the bonds issued hereunder and at the time outstanding, upon payment to the trustee so removed of all moneys then due to it or him hereunder, by an instrument or concurrent instruments in writing, signed in duplicate by such holders. One copy shall be filed with the Company and the other with the trustee so removed.

The Co-Trustee and any successor to the Co-Trustee may be removed at any time by an instrument in writing signed in duplicate by the Trustee, one copy of which shall be filed with the Company and the other delivered to the Co-Trustee so removed.

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In case at any time either of the Trustees or any successor trustee shall resign, die, be dissolved or be removed or otherwise shall become disqualified to act or incapable of acting, or in case control of the Trustee or of any successor trustee, or of its officers shall be taken over by any public officer or officers, a successor trustee may be appointed by the holders of a majority in principal amount of the bonds issued hereunder and at the time outstanding by an instrument or concurrent instruments in writing signed in duplicate by such holders, and filed, one copy with the retiring trustee and the other with the successor trustee, notification thereof being given to the Company by such successor trustee; but until a successor trustee shall be so appointed by the bondholders as herein authorized, the Company, by an instrument in writing, executed by order of the Board of Directors, shall in any such case appoint a successor to the Trustee and the Trustee shall, by an instrument in writing in any such case, appoint a successor to the Co-Trustee. Every such successor to the Trustee so appointed by the bondholders, by a court of competent jurisdiction or by the Company shall be a bank or trust company in good standing organized and doing business under the laws of the United States or of any State, having an office in the United States of America, and (a) which shall be a corporation having a combined capital and surplus of not less than $5,000,000, (b) which shall be authorized under the laws of the jurisdiction of incorporation to exercise corporate trust powers, and (c) which shall be subject to supervision or examination by a Federal or State authority. If such successor Trustee publishes reports of condition at least annually, pursuant to law or to the requirements of such supervising or examining authority, the combined capital and surplus of such successor Trustee shall be deemed to be its combined capital and surplus as set forth in its most recent report of condition so published. Every such successor trustee appointed by the bondholders or by the Trustee in succession to the Co-Trustee shall always be an individual, a citizen of the United States of America, unless otherwise required by law.

Anything hereinabove to the contrary notwithstanding, in case at any time the Co-Trustee, or any successor thereto, shall die, become incapable of acting, resign or be removed, all the estates, properties, rights, powers, trusts, duties and obligations of the Trustees hereunder shall, to the extent permitted by law, vest in and be exercised by the Trustee, without the appointment of a successor Co-Trustee.

If in a proper case no appointment of a successor to the Trustee or of a successor to the Co-Trustee shall be made pursuant to the foregoing provisions of this Article XV within six months after a vacancy shall have occurred in the office of trustee, the holder of any bond or the retiring Trustee or Co-Trustee may apply to any court, State or Federal having jurisdiction to appoint a successor trustee, and such court may thereupon, after such notice, if any, as such court may deem proper and prescribe, appoint a successor to the Trustee or to the Co-Trustee, as the case may be.

The holders of the bonds of Series 2008D and the bonds of Series 2008F shall be deemed to have approved the foregoing amendment; however, the foregoing amendment shall not become effective until such time as it shall have received the requisite approvals under the provisions of the Mortgage.

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ARTICLE IV

CONFIRMATION OF LIEN

The Company, for the equal and proportionate benefit and security of the holders of all bonds at any time issued under the Mortgage, hereby confirms the lien of the Mortgage upon, and hereby grants, bargains, sells, transfers, assigns, pledges, mortgages, warrants and conveys unto the Trustees, all property of the Company and all property hereafter acquired by the Company, other than (in each case) property which, by virtue of any of the provisions of the Mortgage, is excluded from such lien, and hereby confirms the title of the Trustees (as set forth in the Mortgage) in and to all such property. Without in any way limiting or restricting the generality of the foregoing, there is specifically included within the confirmation of lien and title hereinabove expressed the property of the Company legally described on Exhibit C attached hereto and made a part hereof.

ARTICLE V

MISCELLANEOUS

The terms and conditions of this Supplemental Indenture shall be deemed to be a part of the terms and conditions of the Mortgage for any and all purposes. The Mortgage, as supplemented by the indentures supplemental thereto dated subsequent to August 1, 1944 and referred to in the first paragraph of this Supplemental Indenture, and as further supplemented by this Supplemental Indenture, is in all respects hereby ratified and confirmed.

This Supplemental Indenture shall bind and, subject to the provisions of Article XIV of the Mortgage, inure to the benefit of the respective successors and assigns of the parties hereto.

Although this Supplemental Indenture is dated as of April 23, 2008, it shall be effective only from and after the actual time of its execution and delivery by the Company and the Trustees on the date indicated by their respective acknowledgments hereto annexed.

Notwithstanding anything to the contrary contained in the Mortgage, the maximum amount of indebtedness secured by the Mortgage shall not exceed 200% of the aggregate stated principal amount of the bonds of each series presently outstanding under, and secured by, the Mortgage, as set forth in the Recitals to this Supplemental Indenture, except to the extent such maximum amount may be adjusted by a subsequent recorded supplemental indenture (which adjustment, and the corresponding supplemental indenture, shall not require the consent or approval of the holders of any bonds then outstanding under the Mortgage, including the holders of the bonds of Series 2008D and the holders of the bonds of Series 2008F).

This Supplemental Indenture may be simultaneously executed in any number of counterparts, and all such counterparts executed and delivered, each as an original, shall constitute but one and the same instrument.

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IN WITNESS WHEREOF, Commonwealth Edison Company has caused this Supplemental Indenture to be executed in its name by its Senior Vice President, Chief Financial Officer and Treasurer, and attested by its Secretary, and BNY Midwest Trust Company, as Trustee under the Mortgage, has caused this Supplemental Indenture to be executed in its name by one of its Vice Presidents, and attested by one of its Assistant Treasurers, and D.G. Donovan, as Co-Trustee under the Mortgage, has hereunto affixed his signature, all as of the day and year first above written.

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I, MARY E. NOLAN, a Notary Public in and for said County, in the State aforesaid, DO HEREBY CERTIFY that Robert K. McDonald, Senior Vice President, Chief Financial Officer and Treasurer of Commonwealth Edison Company, an Illinois corporation, one of the parties described in and which executed the foregoing instrument, and Donna Massey, Secretary of said corporation, who are both personally known to me to be the same persons whose names are subscribed to the foregoing instrument as such Senior Vice President, Chief Financial Officer and Treasurer and Secretary, respectively, and who are both personally known to me to be Senior Vice President, Chief Financial Officer and Treasurer and Secretary, respectively, of said corporation, appeared before me this day in person and severally acknowledged that they signed, executed and delivered said instrument as their free and voluntary act as such Senior Vice President, Chief Financial Officer and Treasurer and Secretary, respectively, of said corporation, and as the free and voluntary act of said corporation, for the uses and purposes therein set forth.

GIVEN under my hand and notarial seal this 24^th day of April, A.D. 2008.

(NOTARIAL SEAL)

My Commission expires April 23, 2009.

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I, T. MOSTERD, a Notary Public in and for said County, in the State aforesaid, DO HEREBY CERTIFY that J. BARTOLINI, Vice President of BNY Midwest Trust Company, an Illinois trust company, one of the parties described in and which executed the foregoing instrument, and A. HERNANDEZ, Assistant Treasurer of said trust company, who are both personally known to me to be the same persons whose names are subscribed to the foregoing instrument as such Vice President and Assistant Treasurer, respectively, and who are both personally known to me to be a Vice President and an Assistant Treasurer, respectively, of said trust company, appeared before me this day in person and severally acknowledged that they signed, executed and delivered said instrument as their free and voluntary act as such Vice President and Assistant Treasurer of said trust company, and as the free and voluntary act of said trust company, for the uses and purposes therein set forth.

GIVEN under my hand and notarial seal this 23^rd day of April, A.D. 2008.

(NOTARIAL SEAL)

My Commission expires January 22, 2009.

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I, T. MOSTERD, a Notary Public in and for said County, in the State aforesaid, DO HEREBY CERTIFY that D.G. DONOVAN, one of the parties described in and which executed the foregoing instrument, who is personally known to me to be the same person whose name is subscribed to the foregoing instrument, appeared before me this day in person and acknowledged that he signed, executed and delivered said instrument as his free and voluntary act for the uses and purposes therein set forth.

GIVEN under my hand and notarial seal this 23^rd day of April, A.D. 2008.

(NOTARIAL SEAL)

My Commission expires January 22, 2009.

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EXHIBIT A

to

Supplemental Indenture

COMMONWEALTH EDISON COMPANY

First Mortgage Bond, Pollution Control Series 2008D

Due March 1, 2020

COMMONWEALTH EDISON COMPANY, an Illinois corporation (hereinafter called the “Company”), for value received, hereby promises to pay to                                                  , as trustee under that certain Bond Indenture dated as of April 15, 2008 (the “IFA 2008D Indenture”) between Illinois Finance Authority (“IFA”) and said trustee, or registered assigns, on the first day of March, 2020, the sum of                      Dollars, and to pay interest on said sum from the date hereof until said sum shall be paid, at a rate per annum on each day which is equal to the weighted-average interest rate borne on the IFA 2008D Bonds (as defined in the hereinafter referred to Supplemental Indenture) outstanding on such date, until the principal thereof shall be paid in full, subject to Section 2.04(a) of the Supplemental Indenture dated as of April 23, 2008 (the “Supplemental Indenture”), executed and delivered by the Company to the Trustees (as hereinafter defined), which provides for certain credits towards payment of principal of and interest on the bonds of this Series. Interest shall accrue on the bonds of this Series from the date of issuance hereof, and the payment thereof shall be credited as provided in Section 2.04(a)(i) of the Supplemental Indenture unless and until the Trustee receives the notice contemplated by Section 2.04(a)(ii) of the Supplemental Indenture, whereupon the interest on the bonds of this Series shall become and remain due and payable until such time as the Trustee receives a further written notice (including a telex, telegram, telecopy or other form of written telecommunication) from the trustee under the IFA 2008D Indenture stating that such payments need not continue. When interest is due and payable as described above, interest on the bonds of this Series shall be payable at the same time as interest on the IFA 2008D Bonds and upon maturity, redemption, or acceleration of the bonds of this Series, subject to Section 2.04(a) of the Supplemental Indenture. The interest on each bond of this Series so payable on any interest payment date shall, subject to the exceptions provided in Section 3.01 of the Mortgage (as hereinafter defined), as amended by a supplemental indenture dated April 1, 1967, be paid to the person in whose name such bond is registered on the date of such payment. The principal of, and premium, if any, and the interest on, this bond shall be payable at the office or agency of the Company in the City of Chicago, State of Illinois in any coin or currency of the United States of America which at the time of payment is legal tender for the payment of public and private debts.

This bond is one of the bonds of the Company, issued and to be issued in series from time to time under and in accordance with and, irrespective of the time of issue, equally and ratably secured by the Mortgage dated July 1, 1923, and indentures supplemental thereto, under which BNY Midwest Trust Company and D.G. Donovan (collectively, the “Trustees”) are now

A-1

the Trustees, and is one of the First Mortgage Bonds, Pollution Control Series 2008D of the Company, the issuance of which is provided for by the Supplemental Indenture, executed and delivered by the Company to such Trustees, to which Mortgage and all indentures supplemental thereto reference is hereby made for a description of the property mortgaged and pledged, the nature and extent of the security, the rights of the holders and registered owners of said bonds, of the Company and of the Trustees in respect of the security, and the terms and conditions governing the issuance and security of said bonds. The term “Mortgage,” as hereinafter used, shall mean said Mortgage dated July 1, 1923, and all indentures supplemental thereto.

With the consent of the Company and to the extent permitted by and as provided in the Mortgage, modifications or alterations of the Mortgage or of any indenture supplemental thereto and of the rights and obligations of the Company and of the holders and registered owners of the bonds may be made, and compliance with any provision of the Mortgage or any such supplemental indenture may be waived, by the affirmative vote of the holders and registered owners of not less than eighty per centum (80%) in principal amount of the bonds then outstanding under the Mortgage, and by the affirmative vote of the holders and registered owners of not less than eighty per centum (80%) in principal amount of the bonds of any series then outstanding under the Mortgage 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 Mortgage are so affected, but in any case excluding bonds disqualified from voting by reason of the Company’s interest therein as provided in the Mortgage; subject, however, to the condition, among other conditions stated in the Mortgage, that no such modification or alteration shall be made which will permit the extension of the time or times of payment of the principal of or the interest or the premium, if any, on this bond, or the reduction in the principal amount hereof or in the rate of interest or the amount of any premium hereon, or any other modification in the terms of payment of such principal, interest or premium, which terms of payment are unconditional, or, otherwise than as permitted by the Mortgage, the creation of any lien ranking prior to or on a parity with the lien of the Mortgage with respect to any of the mortgaged property, all as more fully provided in the Mortgage.

The bonds of this Series are subject to redemption, as provided in the Supplemental Indenture.

In case of certain completed defaults specified in the Mortgage, the principal of this bond may be declared or may become due and payable in the manner and with the effect provided in the Mortgage.

No recourse shall be had for the payment of the principal of or the interest on this bond, or for any claim based hereon, or otherwise in respect hereof or of the Mortgage, to or against any incorporator, stockholder, officer or director, past, present or future, of the Company or of any successor corporation, either directly or through the Company or such successor corporation, under any constitution or statute or rule of law, or by the enforcement of any assessment or penalty, or otherwise, all such liability of incorporators, stockholders, directors and officers being waived and released by the registered owner hereof by the acceptance of this bond and being likewise waived and released by the terms of the Mortgage, all as more fully provided therein.

A-2

This bond is transferable by the registered owner hereof, in person or by duly authorized attorney, at the office or agency of the Company in the City of Chicago, State of Illinois, upon surrender and cancellation of this bond; and thereupon a new registered bond or bonds without coupons of the same aggregate principal amount and series will, upon the payment of charges as provided in the Mortgage, be issued to the transferee in exchange herefor.

Bonds of this Series are issuable only in registered form without coupons and in the denominations of $1,000 each and any authorized multiple thereof. As provided in the Mortgage, such bonds are exchangeable for registered bonds of the same series as between authorized denominations. Any such exchange may be made by the registered owner of any such bond or bonds upon presentation thereof for that purpose at the office or agency of the Company in the City of Chicago, State of Illinois.

This bond shall not be entitled to any security or benefit under the Mortgage or be valid or become obligatory for any purpose unless and until it shall have been authenticated by the execution by the corporate Trustee, or its successor in trust under the Mortgage, of the certificate endorsed hereon.

A-3

IN WITNESS WHEREOF, Commonwealth Edison Company has caused this bond to be executed in its name by its President or one of its Vice Presidents, and has caused its corporate seal to be hereto affixed, attested by its Secretary or one of its Assistant Secretaries, as of the          day of                     , 20    .

[SEAL]

(General Form of Trustee’s Certificate)

This bond is one of the bonds of the series designated herein, referred to and described in the within mentioned Supplemental Indenture dated as of April 23, 2008.

Illinois Commerce Commission Identification No.

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ABBREVIATIONS

The following abbreviations, when used in the inscription on the face of this instrument, shall be construed as though they were written out in full according to applicable laws or regulations:

Additional abbreviations may also be used though not in the above list.

FOR VALUE RECEIVED the undersigned hereby sell(s), assign(s), and transfer(s) unto

(Please print or typewrite name and address including postal zip code of assignee)

the within Bond and all rights thereunder, hereby irrevocably constituting and appointing                                          attorney to transfer said Bond on the books of the Company, with full power of substitution in the premises.

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EXHIBIT B

to

Supplemental Indenture

COMMONWEALTH EDISON COMPANY

First Mortgage Bond, Pollution Control Series 2008F

Due March 1, 2017

COMMONWEALTH EDISON COMPANY, an Illinois corporation (hereinafter called the “Company”), for value received, hereby promises to pay to                                                  , as trustee under that certain Bond Indenture dated as of April 15, 2008 (the “IFA 2008F Indenture”) between Illinois Finance Authority (“IFA”) and said trustee, or registered assigns, on the first day of March, 2017, the sum of                      Dollars, and to pay interest on said sum from the date hereof until said sum shall be paid, at a rate per annum on each day which is equal to the weighted-average interest rate borne on the IFA 2008F Bonds (as defined in the hereinafter referred to Supplemental Indenture) outstanding on such date, until the principal thereof shall be paid in full, subject to Section 2.04(b) of the Supplemental Indenture dated as of April 23, 2008 (the “Supplemental Indenture”), executed and delivered by the Company to the Trustees (as hereinafter defined), which provides for certain credits towards payment of principal of and interest on the bonds of this Series. Interest shall accrue on the bonds of this Series from the date of issuance hereof, and the payment thereof shall be credited as provided in Section 2.04(b)(i) of the Supplemental Indenture unless and until the Trustee receives the notice contemplated by Section 2.04(b)(ii) of the Supplemental Indenture, whereupon the interest on the bonds of this Series shall become and remain due and payable until such time as the Trustee receives a further written notice (including a telex, telegram, telecopy or other form of written telecommunication) from the trustee under the IFA 2008F Indenture stating that such payments need not continue. When interest is due and payable as described above, interest on the bonds of this Series shall be payable at the same time as interest on the IFA 2008F Bonds and upon maturity, redemption, or acceleration of the bonds of this Series, subject to Section 2.04(b) of the Supplemental Indenture. The interest on each bond of this Series so payable on any interest payment date shall, subject to the exceptions provided in Section 3.01 of the Mortgage (as hereinafter defined), as amended by a supplemental indenture dated April 1, 1967, be paid to the person in whose name such bond is registered on the date of such payment. The principal of, and premium, if any, and the interest on, this bond shall be payable at the office or agency of the Company in the City of Chicago, State of Illinois in any coin or currency of the United States of America which at the time of payment is legal tender for the payment of public and private debts.

This bond is one of the bonds of the Company, issued and to be issued in series from time to time under and in accordance with and, irrespective of the time of issue, equally and ratably secured by the Mortgage dated July 1, 1923, and indentures supplemental thereto, under which BNY Midwest Trust Company and D.G. Donovan (collectively, the “Trustees”) are now

B-1

the Trustees, and is one of the First Mortgage Bonds, Pollution Control Series 2008F of the Company, the issuance of which is provided for by the Supplemental Indenture, executed and delivered by the Company to such Trustees, to which Mortgage and all indentures supplemental thereto reference is hereby made for a description of the property mortgaged and pledged, the nature and extent of the security, the rights of the holders and registered owners of said bonds, of the Company and of the Trustees in respect of the security, and the terms and conditions governing the issuance and security of said bonds. The term “Mortgage,” as hereinafter used, shall mean said Mortgage dated July 1, 1923, and all indentures supplemental thereto.

With the consent of the Company and to the extent permitted by and as provided in the Mortgage, modifications or alterations of the Mortgage or of any indenture supplemental thereto and of the rights and obligations of the Company and of the holders and registered owners of the bonds may be made, and compliance with any provision of the Mortgage or any such supplemental indenture may be waived, by the affirmative vote of the holders and registered owners of not less than eighty per centum (80%) in principal amount of the bonds then outstanding under the Mortgage, and by the affirmative vote of the holders and registered owners of not less than eighty per centum (80%) in principal amount of the bonds of any series then outstanding under the Mortgage 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 Mortgage are so affected, but in any case excluding bonds disqualified from voting by reason of the Company’s interest therein as provided in the Mortgage; subject, however, to the condition, among other conditions stated in the Mortgage, that no such modification or alteration shall be made which will permit the extension of the time or times of payment of the principal of or the interest or the premium, if any, on this bond, or the reduction in the principal amount hereof or in the rate of interest or the amount of any premium hereon, or any other modification in the terms of payment of such principal, interest or premium, which terms of payment are unconditional, or, otherwise than as permitted by the Mortgage, the creation of any lien ranking prior to or on a parity with the lien of the Mortgage with respect to any of the mortgaged property, all as more fully provided in the Mortgage.

The bonds of this Series are subject to redemption, as provided in the Supplemental Indenture.

In case of certain completed defaults specified in the Mortgage, the principal of this bond may be declared or may become due and payable in the manner and with the effect provided in the Mortgage.

No recourse shall be had for the payment of the principal of or the interest on this bond, or for any claim based hereon, or otherwise in respect hereof or of the Mortgage, to or against any incorporator, stockholder, officer or director, past, present or future, of the Company or of any successor corporation, either directly or through the Company or such successor corporation, under any constitution or statute or rule of law, or by the enforcement of any assessment or penalty, or otherwise, all such liability of incorporators, stockholders, directors and officers being waived and released by the registered owner hereof by the acceptance of this bond and being likewise waived and released by the terms of the Mortgage, all as more fully provided therein.

B-2

This bond is transferable by the registered owner hereof, in person or by duly authorized attorney, at the office or agency of the Company in the City of Chicago, State of Illinois, upon surrender and cancellation of this bond; and thereupon a new registered bond or bonds without coupons of the same aggregate principal amount and series will, upon the payment of charges as provided in the Mortgage, be issued to the transferee in exchange herefor.

Bonds of this Series are issuable only in registered form without coupons and in the denominations of $1,000 each and any authorized multiple thereof. As provided in the Mortgage, such bonds are exchangeable for registered bonds of the same series as between authorized denominations. Any such exchange may be made by the registered owner of any such bond or bonds upon presentation thereof for that purpose at the office or agency of the Company in the City of Chicago, State of Illinois.

This bond shall not be entitled to any security or benefit under the Mortgage or be valid or become obligatory for any purpose unless and until it shall have been authenticated by the execution by the corporate Trustee, or its successor in trust under the Mortgage, of the certificate endorsed hereon.

B-3

IN WITNESS WHEREOF, Commonwealth Edison Company has caused this bond to be executed in its name by its President or one of its Vice Presidents, and has caused its corporate seal to be hereto affixed, attested by its Secretary or one of its Assistant Secretaries, as of the          day of                     , 20    .

[SEAL]

(General Form of Trustee’s Certificate)

This bond is one of the bonds of the series designated herein, referred to and described in the within mentioned Supplemental Indenture dated as of April 23, 2008.

Illinois Commerce Commission Identification No.

B-4

ABBREVIATIONS

The following abbreviations, when used in the inscription on the face of this instrument, shall be construed as though they were written out in full according to applicable laws or regulations:

Additional abbreviations may also be used though not in the above list.

FOR VALUE RECEIVED the undersigned hereby sell(s), assign(s), and transfer(s) unto

(Please print or typewrite name and address including postal zip code of assignee)

the within Bond and all rights thereunder, hereby irrevocably constituting and appointing                                          attorney to transfer said Bond on the books of the Company, with full power of substitution in the premises.

B-5

EXHIBIT C

to

Supplemental Indenture

Legal Descriptions

[omitted]

C-1