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                                                                   Exhibit 10.91
                                                           EXECUTION COUNTERPART

                             SUPPLEMENTAL AGREEMENT

            SUPPLEMENTAL AGREEMENT (this "SUPPLEMENTAL AGREEMENT") dated as
of May 30, 2001 by and among EDISON MISSION ENERGY (the "BORROWER"), BANK OF
AMERICA, N.A. ("BOA"), as agent for the 425 Lenders (as defined below) and as
administrative agent for the 255 Lenders (as defined below) and CITICORP USA,
INC. ("CUSA") as administrative agent for the 595 Lenders (as defined below).

            WHEREAS, (a) the Borrower, certain of the lenders party thereto
(the "425 LENDERS") and BOA (formerly, the Bank of America National Trust and
Savings Association), as agent (the "425 AGENT") entered into the Second
Amended and Restated Credit Agreement dated as of October 11, 1996 (as
amended by Amendment One dated as of August 17, 2000 among the Borrower, the
425 Agent and the 425 Lenders party thereto, the "425 CREDIT AGREEMENT"); (b)
the Borrower, certain of the lenders party thereto (the "255 LENDERS") and
BOA as administrative agent (the "255 AGENT") entered into the Credit
Agreement dated as of May 30, 2000 (as amended by Amendment One dated as of
August 17, 2000 among the Borrower, the 255 Agent and the 255 Lenders party
thereto, the "255 CREDIT AGREEMENT"); and (c) the Borrower, certain of the
lenders party thereto (the "595 LENDERS", together with the 425 Lenders and
the 255 Lenders, the "LENDERS"), and CUSA as administrative agent (the "595
AGENT", together with the 425 Agent and the 255 Agent, the "AGENTS") entered
into a Credit Agreement dated as of March 18, 1999 (as amended by (i)
Amendment One dated as of August 17, 2000 among the Borrower, the 595 Agent
and the 595 Lenders party thereto and (ii) Amendment Two dated as of March
15, 2001 among the Borrower, the 595 Agent and the 595 Lenders party thereto,
the "595 CREDIT AGREEMENT"). The 425 Credit Agreement, the 255 Credit
Agreement and the 595 Credit Agreement as amended or modified from time to
time are herein referred to as the "CREDIT AGREEMENTS";

            WHEREAS, to induce the Lenders to enter into Amendment Two to the
425 Credit Agreement, Amendment Two to the 255 Credit Agreement and Amendment
Three to the 595 Credit Agreement, each dated as of the date hereof, the
Borrower has agreed to supplement each of the Credit Agreements as further
set forth in this Supplemental Agreement;

            ACCORDINGLY, the parties hereto agree as follows:

            Section 1. DEFINITIONS. Except as otherwise defined in this
Supplemental Agreement, terms defined in the Credit Agreements are used
herein (and in the introductions and recitals hereto) as defined therein.

            Section 2. LOAN DOCUMENT. The Borrower agrees that this
Supplemental Agreement shall be included in the definition of "LOAN DOCUMENT"
for the purposes of each of the Credit Agreements.

            Section 3. ADDITIONAL COVENANT. The Borrower shall not declare,
pay or make, or agree to declare, pay or make, either directly or indirectly,
Covered Distributions during the

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period beginning on and including May 29, 2001 and ending on and including
October 10, 2001 in an aggregate amount exceeding $32,500,000. For the
purposes of this section, a "COVERED DISTRIBUTION" means (a) any dividend
(other than dividends payable solely in the common stock of the Borrower) on,
or any payment on account of, or any setting apart of assets for a sinking or
other analogous fund for, the purchase, redemption, defeasance, retirement or
other acquisition of, any shares of any class of capital stock of the
Borrower or any warrants or options to purchase any such stock, whether now
or hereafter outstanding, or any other distribution in respect thereof and
(b) any payment of the principal of, interest on or other amounts payable
with respect to intercompany loans to the Borrower from Edison International
or any subsidiary of Edison International that is not a subsidiary of the
Borrower.

            Section 4. ADDITIONAL EVENTS OF DEFAULT. The Borrower agrees that
the following events or occurrences shall constitute an "Event of Default"
under, and as defined in, each of the Credit Agreements:

            (a) The Borrower shall default in the performance and observance
of any of its obligations under Section 3 above; or

            (b) Any Change of Control shall occur. For purposes of this
section, "CHANGE OF CONTROL" means the failure of Edison International to
own, directly or indirectly, at least 50.1% of the outstanding shares of
voting stock of the Borrower (or any successor permitted pursuant to the
Credit Agreements) on a fully diluted basis.

            Section 5. PERIODS OF EFFECTIVENESS. The provisions of this
Supplemental Agreement shall be effective as to each Credit Agreement until
such Credit Agreement shall be fully and finally paid in full; PROVIDED that
the provisions of Sections 4(b) above shall cease to be effective as to each
Credit Agreement (and the events or occurrences set forth in Section 4(b)
above shall cease to be an Event of Default under each Credit Agreement) in
the event that (a) the Borrower shall have furnished to the Lenders a binding
financing commitment for a financing in an aggregate principal amount of at
least $275,000,000 that is in form and substance acceptable to the Requisite
COC Lenders from one or more banks or other financial institutions that are
acceptable to the Requisite COC Lenders the proceeds of which, when received
by the Borrower, shall be ratably applied to prepay permanently outstanding
loans under the Credit Agreements or (b) outstanding loans and commitments
under the Credit Agreements have been ratably reduced by at least
$275,000,000 by the application of the Net Cash Proceeds from the issuance or
incurrence by the Borrower of indebtedness. For purposes of this Supplemental
Agreement, "REQUISITE COC LENDERS" means (a) Required Lenders under, and as
defined in, each Credit Agreement AND (b) Lenders under the 595 Credit
Agreement that have no outstanding extensions of credit to Edison
International holding 85% of the aggregate outstanding principal amount of
the loans of such Lenders under the 595 Credit Agreement (or, if no such
loans are outstanding, 85% of the aggregate commitments of such Lenders under
the 595 Credit Agreement).

            Section 6. MODIFICATION. The provisions of this Supplemental
Agreement may only be amended, modified, waived or supplemented with the
prior written consent of (a) the Borrower, (b) the 425 Agent, acting at the
direction of the Requisite Lenders, (c) the 255 Agent, acting at the
direction of the Requisite Lenders and (d) the 595 Agent, acting at the
direction of

                               SUPPLEMENTAL AGREEMENT
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the Requisite Lenders. For purposes of this Section, "REQUISITE LENDERS"
means, in the case of amendments, modifications, waivers or supplements to
(a) Section 3 or 4(a), Section 5 (as it may relate to Section 3 or 4(a)),
this Section 6(a) or any defined term used in said Sections, 100% of the
Lenders, (b) Section 4(b), Section 5 (as it may relate to Section 4(b)), this
Section 6(b) or any defined term used in said Sections, Requisite COC Lenders
and (c) any provision in this Supplemental Agreement not referred to in
clauses (a) or (b) above, Required Lenders under, and as defined in, each
Credit Agreement.

            Section 7. MISCELLANEOUS. This Supplemental Agreement 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 Supplemental Agreement by signing any such counterpart. This
Supplemental Agreement shall be governed by, and construed in accordance
with, the law of the State of New York.



















                               SUPPLEMENTAL AGREEMENT
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            IN WITNESS WHEREOF, the parties hereto have caused this
Supplemental Agreement to be executed and delivered by their respective
officers thereunto duly authorized.

                                    EDISON MISSION ENERGY



                                    By: /s/  G. Gary Garcia
                                       ---------------------------------------
                                       Name:  G. Gary Garcia
                                       Title: Treasurer

                                    BANK OF AMERICA, N.A.,
                                      as 425 Agent and as 255 Agent



                                    By: /s/ David Price
                                       ---------------------------------------
                                       Name:  David Price
                                       Title: Vice President

                                    CITICORP USA, INC.,
                                       as 595 Agent



                                    By: /s/  Anita J. Brickell
                                       ---------------------------------------
                                       Name:  Anita J. Brickell
                                       Title: Director