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                                                   Exhibit (4)-25
                                                   Unicom Corporation
                                                   Form 10-K  File No. 1-11375


                                    GUARANTY


     GUARANTY, dated as of December 17, 1999, made by UNICOM CORPORATION, a
corporation organized and existing under the laws of the State of Illinois (the
"Guarantor"), in favor of the Lenders (the "Lenders"), the LC Issuer identified
hereunder and Bank One, NA, as agent (in such capacity, the "Agent") for the
Lenders.


                             PRELIMINARY STATEMENTS

     (1)  The Lenders and the Agent have entered into a Credit Agreement, dated
as of the date hereof (said Agreement, as it may hereafter be amended or
otherwise modified from time to time, being the "Credit Agreement", the terms
defined therein and not otherwise defined herein being used herein as therein
defined), with Unicom Enterprises Inc., a corporation organized and existing
under the laws of the State of Illinois (the "Borrower"). The Guarantor will
derive substantial direct and indirect benefit from the transactions
contemplated by the Credit Agreement and the other Loan Documents.

     (2)  The Borrower is a wholly-owned Subsidiary of the Guarantor.

     (3)  It is a condition precedent to the making of Advances by the Lenders
under the Credit Agreement and the issuance of Facility LCs by the LC Issuer
pursuant to the Credit Agreement that the Guarantor shall have executed and
delivered this Guaranty.

     NOW, THEREFORE, in consideration of the premises and in order to induce the
Lenders to make Advances and the LC Issuer to issue Facility LCs under the
Credit Agreement, the Guarantor hereby agrees as follows:

     SECTION 1. Certain Defined Terms. As used in this Guaranty, the following
terms shall have the following meanings (such meanings to be equally applicable
to both the singular and plural forms of the terms defined):

          "Consolidated Capitalization" means, with respect to any Person, at
     any date of determination, the sum of (a) Consolidated Indebtedness of such
     Person, (b) consolidated equity of the common stockholders of such Person
     and its Consolidated Subsidiaries, (c) consolidated equity of the
     preference stockholders of such Person and its Consolidated Subsidiaries,
     (d) consolidated equity of the preferred stockholders of such Person and
     its Consolidated Subsidiaries and (e) the aggregate principal amount of
     Subordinated Deferrable Interest Securities of such Person and its
     Consolidated Subsidiaries, in each case determined at such date in
     accordance with GAAP.
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          "Consolidated Indebtedness" means, with respect to any Person, at any
     date of determination, the aggregate Indebtedness of such Person and its
     Consolidated Subsidiaries determined on a consolidated basis in accordance
     with GAAP, but shall not include (i) Nonrecourse Indebtedness of any
     Subsidiary of the Guarantor, (ii) the aggregate principal amount of
     Subordinated Deferrable Interest Securities of such Person and its
     Consolidated Subsidiaries and (iii) the aggregate principal amount of
     Transitional Funding Instruments of such Person and its Consolidated
     Subsidiaries.

          "Consolidated Subsidiary" means, as to any Person, any Subsidiary of
     such Person whose accounts are or are required to be consolidated with the
     accounts of such Person in accordance with GAAP.

          "Consolidated Tangible Net Worth" means, at any time of determination,
     with respect to any Person and its Consolidated Subsidiaries, the excess of
     such Person's and such Person's Consolidated Subsidiaries' total assets
     over its total liabilities, with total assets and total liabilities each to
     be determined in accordance with GAAP consistently applied, excluding,
     however, from the determination of total assets (i) goodwill,
     organizational expenses, research and development expenses, trademarks,
     trade names, copyrights, patents, patent applications, licenses and rights
     in any thereof, and other similar intangibles, (ii) all prepaid expenses,
     deferred charges or unamortized indebtedness discount and expense, (iii)
     all reserves carried and not deducted from assets, (iv) securities that are
     not readily marketable, (v) cash held in a sinking or other analogous fund
     established for the purpose of redemption, retirement or prepayment of
     capital stock or Indebtedness, (vi) any write-up in the book value of any
     asset resulting from a revaluation thereof subsequent to September 30,
     1994, and (vii) any items not included in clauses (i) through (vi), above,
     that are treated as intangibles in conformity with GAAP.

          "Contingent Obligation" means, as to any Person, the undrawn face
     amount of any letters of credit issued for the account of such Person and
     shall also mean any monetary obligation of such Person guaranteeing or in
     effect guaranteeing any Indebtedness, leases, dividends, letters of credit,
     or other obligations ("primary obligations") of any other Person (the
     "primary obligor") in any manner, whether directly or indirectly,
     including, without limitation, any obligation of such Person, whether or
     not contingent, (a) to purchase any such primary obligation or any property
     constituting direct or indirect security therefor, (b) to advance or supply
     funds (i) for the purchase or payment of any such primary obligation or
     (ii) to maintain working capital or equity capital of the primary obligor
     or otherwise to maintain the net worth or solvency of the primary obligor,
     (c) to purchase property, securities, or services primarily for the purpose
     of assuring the obligee under any such primary obligation of the ability of
     the primary obligor to make payment of such primary obligation, or (d)
     otherwise to assure or hold harmless the obligee under such primary
     obligation against loss in respect thereof; provided, however, that the
     term Contingent Obligation shall not include endorsements of instruments
     for deposit or collection in the ordinary course of business. The amount of
     any Contingent Obligation shall be deemed to be an amount equal to the
     stated or determinable amount of the primary obligation or, where such
     Contingent Obligation is specifically limited to a portion of any such
     primary obligation, that portion to which it is
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     limited or, if not stated or determinable, the maximum reasonably
     anticipated liability in respect thereof (assuming such Person is required
     to perform thereunder) as determined by such Person in good faith. For
     purposes of computing the Consolidated Indebtedness of any Person, the
     amount of any primary obligation of any Subsidiary of such Person and the
     amount of any Contingent Obligation of such Person or any Subsidiary of
     such Person corresponding to such primary obligation shall only be counted
     once (i.e., without duplication).

          "Subordinated Deferrable Interest Securities" means all obligations of
     the Guarantor and its Subsidiaries in respect of "ComEd- Obligated
     Mandatorily Redeemable Preferred Securities of Subsidiary Trusts", as set
     forth from time to time in the consolidated balance sheets of the Guarantor
     and its Consolidated Subsidiaries delivered pursuant to Section 7(a)
     hereof.

          "Transitional Funding Instruments" means any instruments, pass-through
     certificates, notes, debentures, certificates of participation, bonds,
     certificates of beneficial interest or other evidences of indebtedness or
     instruments evidencing a beneficial interest which (i) are issued pursuant
     to a "transitional funding order" (as such term is defined in Section 18-
     102 of the Illinois Public Utilities Act, as amended) issued by the
     Illinois Commerce Commission at the request of an electric utility and (ii)
     are secured by or otherwise payable from non-bypassable cent per kilowatt
     hour charges authorized pursuant to such order to be applied and invoiced
     to customers of such utility. The instrument funding charges so applied and
     invoiced must be deducted and stated separately from the other charges
     invoiced by such utility against its customers.

     SECTION 2. Guaranty. The Guarantor hereby absolutely, unconditionally and
irrevocably guaranties the punctual payment when due, without setoff,
counterclaim or other deduction, whether at stated maturity, by acceleration or
otherwise, of all obligations of the Borrower now or hereafter existing under
the Credit Agreement, the Notes and any other Loan Documents, whether for
principal, reimbursement obligations, interest, fees, expenses or otherwise (all
such obligations being the "Obligations"), and agrees to pay any and all
expenses (including counsel fees and expenses) incurred by the Agent, the LC
Issuer or the Lenders in enforcing any rights under this Guaranty. Without
limiting the generality of the foregoing, the Guarantor's liability shall extend
to all amounts that constitute part of the Obligations and would be owed by the
Borrower to the Agent, the LC Issuer or the Lenders under the Credit Agreement,
the Notes and the other Loan Documents but for the fact that they are
unenforceable or not allowable due to the existence of a bankruptcy,
reorganization or similar proceeding involving the Borrower.

     SECTION 3. Guaranty Absolute. The Guarantor guarantees that the Obligations
will be paid strictly in accordance with the terms of the Credit Agreement, the
Notes and the other Loan Documents, regardless of any law, regulation or order
now or hereafter in effect in any jurisdiction affecting any of such terms or
the rights of the Agent, the LC Issuer or the Lenders with respect thereto. The
obligations of the Guarantor under this Guaranty are independent of the
Obligations, and a separate action or actions may be brought and prosecuted
against the Guarantor to enforce this Guaranty, irrespective of whether any
action is brought against the
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Borrower or whether the Borrower is joined in any such action or actions. The
liability of the Guarantor under this Guaranty shall be absolute, unconditional
and irrevocable irrespective of:

          (i)  any lack of validity or enforceability of the Credit Agreement,
     the Notes, any other Loan Document, any Advance, or any other agreement or
     instrument relating thereto;

          (ii)  any change in the time, manner or place of payment of, or in any
     other term of, all or any of the Obligations, or any other amendment or
     waiver of, or any consent to departure from, the Credit Agreement, the
     Notes or any other Loan Document, including, without limitation, any
     increase in the Obligations resulting from the extension of additional
     credit to the Borrower or otherwise and any extension of the Facility
     Termination Date;

          (iii)  any manner of application of collateral, or proceeds thereof,
     to all or any of the Obligations, or any manner of sale or other
     disposition of, any release or impairment of, or any failure to perfect,
     any lien on or security interest in any collateral for all or any of the
     Obligations or any other assets of the Borrower or any of its Subsidiaries,
     or any release or discharge of any Person liable for any or all of the
     Obligations;

          (iv)  any change, restructuring or termination of the corporate
     structure or existence of the Borrower or any of its Subsidiaries or any
     bankruptcy, insolvency, liquidation or similar proceeding instituted by or
     against the Borrower or any of its Subsidiaries; or

          (v)  any other circumstance that might otherwise constitute a defense
     available to, or a discharge of, the Borrower or a guarantor.

As against the Guarantor, this Guaranty shall continue to be effective or be
reinstated, as the case may be, if at any time any payment of any of the
Obligations is rescinded or must otherwise be returned by the Agent, any LC
Issuer or any Lender upon the insolvency, bankruptcy or reorganization of the
Borrower or otherwise, all as though such payment had not been made.

     SECTION 4. Waiver. The Guarantor hereby waives promptness, diligence,
presentment, protest, notice of protest, notice of dishonor, notice of
acceptance and any other notice with respect to any of the Obligations and this
Guaranty and any requirement that the Agent, any LC Issuer or any Lender
protect, secure, perfect or insure any security interest or lien on any property
subject thereto or exhaust any right or take any action against the Borrower or
any other person or entity or any collateral.

     SECTION 5. Waiver of Rights of Subrogation. The Guarantor hereby expressly
and irrevocably waives with respect to the Borrower and its successors and
assigns and any other Person, any and all rights at law or in equity, by
agreement or otherwise, to subrogation, reimbursement, exoneration,
contribution, setoff, share in any collateral or any other rights that could
accrue to a surety against a principal, to a guarantor against a maker or
obligor, to an accommodation party against the party accommodated, or to a
holder or transferee against a
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maker, and that the Guarantor may have or hereafter acquire against the
Borrower, any of its Affiliates, or any other Person in connection with or as a
result of the Guarantor's execution, delivery or performance hereunder. In
furtherance of the foregoing, the Guarantor agrees that any payment by the
Guarantor to the Agent, the LC Issuer or the Lenders pursuant to this Guaranty
shall be deemed a contribution to the capital of the Borrower, and no such
payment shall constitute the Guarantor a creditor of the Borrower. The Guarantor
hereby acknowledges and agrees that the foregoing waivers are intended to
benefit the Borrower, the Agent, the LC Issuer and the Lenders and shall not
limit or otherwise affect the Guarantor's liability hereunder or the
enforceability hereof. If, notwithstanding the foregoing, any amount shall be
paid to the Guarantor on account of such subrogation rights at any time when all
of the Obligations shall not have been paid in full, such amount shall be held
by the Guarantor in trust for the Agent, the LC Issuer and the Lenders,
segregated from other funds of the Guarantor, and shall, forthwith upon receipt
by the Guarantor, be turned over to the Agent in the exact form received by the
Guarantor (duly endorsed by the Guarantor to the Agent), to be applied against
the Obligations, whether matured or unmatured, in such order as the Agent may
determine.

     SECTION 6. Representations and Warranties. The Guarantor hereby represents
and warrants as follows:

     (a)  Corporate Existence and Power. It is a corporation duly incorporated,
validly existing and in good standing under the laws of the State of Illinois,
or, following the consummation of the Unicom/PECO Merger, the laws of the
Commonwealth of Pennsylvania, is duly qualified to do business as a foreign
corporation in, and is in good standing under the laws of, each state in which
the ownership of its properties or the conduct of its business makes such
qualification necessary, except where the failure to be so qualified would not
have a Material Adverse Effect, and has all corporate powers and all
governmental licenses, authorizations, consents and approvals required to own or
lease its property and to carry on its business as now conducted.

     (b)  Corporate Authorization. The execution, delivery and performance by it
of this Guaranty have been duly authorized by all necessary corporate action on
its part and do not, and will not, require the consent or approval of its
shareholders, or any trustee or holder of any Indebtedness or other obligation
of the Guarantor.

     (c)  No Violation, Etc. The execution and delivery by the Guarantor of this
Guaranty, and the performance by the Guarantor of its obligations hereunder, (i)
are within the Guarantor's corporate powers, (ii) have been duly authorized by
all necessary corporate action and (iii) do not and will not (A) violate any
provision of the charter or by-laws of the Guarantor or of law, (B) violate any
legal restriction binding on or affecting the Guarantor, (C) result in a breach
of, or constitute a default under, any indenture or loan or credit agreement or
any other agreement, lease or instrument to which the Guarantor is a party or by
which it or its properties may be bound or affected, or (D) result in or require
the creation of any lien or security interest upon or with respect to any of its
properties.

     (d)  Governmental Actions. No authorization or approval or other action by,
and no notice to or filing with, any governmental authority or regulatory body
is required for the due execution, delivery and performance by the Guarantor of
this Guaranty, other than the Merger
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Approvals, which are required for the performance by the Guarantor of this
Guaranty at all times on and after the date of the Unicom/PECO Merger. The
Merger Approvals are and shall be in full force and effect on and after the
effective date of the Unicom/PECO Merger.

     (e)  Execution and Delivery. This Guaranty has been duly executed and
delivered by the Guarantor, and is the legal, valid and binding obligation of
the Guarantor enforceable against it in accordance with its terms, subject,
however, to the application by a court of general principles of equity and to
the effect of any applicable bankruptcy, insolvency, reorganization, moratorium
or similar laws affecting creditors' rights generally.

     (f)  Litigation. There is no pending or threatened action or proceeding
(including, without limitation, any proceeding relating to, or arising out of,
any Environmental Laws) affecting it or any of its Subsidiaries before any
court, governmental agency or arbitrator, that might reasonably be expected to
result in a Material Adverse Effect, or that questions the validity or
enforceability of this Guaranty or any other Loan Document against the Guarantor
or the Borrower.

     (g)  Financial Statements; Material Adverse Change. The consolidated
balance sheet of the Guarantor and its Consolidated Subsidiaries as at September
30, 1999 and the related consolidated statement of income, retained earnings and
cash flows for the period then ended, together with the report thereon of Arthur
Andersen LLP included in the Guarantor's Quarterly Report on Form 10-Q for the
quarterly period ended September 30, 1999, copies of which have been furnished
to each Lender, fairly present the financial condition of the Guarantor and its
Consolidated Subsidiaries as at such date and the results of operations of the
Guarantor and its Consolidated Subsidiaries for the period ended on such date,
all in accordance with GAAP. Since September 30, 1999, there has been no
material adverse change in the financial condition, results of operations,
operations, business or Property of the Guarantor and its Subsidiaries, taken as
a whole, or the Borrower and its Subsidiaries, taken as a whole (other than
operating losses resulting from start-up operations of Subsidiaries of the
Borrower), or in the Guarantor's ability to perform any of its obligations
hereunder.

     (h)  ERISA. No ERISA Event has occurred or is reasonably expected to occur
with respect to any Plan of the Guarantor or any of its ERISA Affiliates which
would result in a liability of $25,000,000 or more to the Guarantor. Since the
most recent September 30 for which financial statements have been delivered to
the Lenders in accordance with Section 7(a) hereof, there has been no material
adverse change in the funding status of the Plans and no "prohibited
transaction" has occurred with respect thereto which is in either event
reasonably expected to result in a liability of $25,000,000 or more to the
Guarantor.

     (i)  Taxes. The Guarantor and each of its Subsidiaries have filed all tax
returns (federal, state and local) required to be filed and paid all taxes shown
thereon to be due, including interest and penalties, or provided adequate
reserves for payment thereof other than such taxes that the Guarantor or such
Subsidiary is contesting in good faith by appropriate legal proceedings and in
respect of which the Guarantor or such Subsidiary, as the case may be, has
established adequate reserves in conformity with GAAP.
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     (j)  Violation of Law. Neither the Guarantor nor any of its Subsidiaries is
in violation of any law or governmental regulation or court decree or order
which might reasonably be expected to result in a Material Adverse Effect.

     (k)  Investment Company. The Guarantor is not an "investment company" or a
company "controlled" by an "investment company" within the meaning of the
Investment Company Act of 1940, as amended, or an "investment advisor" within
the meaning of the Investment Advisers Act of 1940, as amended.

     (l)  Holding Company. The Guarantor is a "holding company" within the
meaning of the Public Utility Holding Company Act of 1935, as amended, but the
Guarantor and its Subsidiaries are exempt from the provisions of that Act,
except Section 9(a)(2) thereof, by virtue of an order issued by the Securities
and Exchange Commission on July 22, 1994. Such exemption is in full force and
effect and, except for proceedings in connection with the transactions
contemplated by the Unicom/PECO Merger Agreement, the Guarantor is not aware of
any existing or proposed proceedings contemplating the revocation or
modification of such exemption.

     (m)  Information. All factual information heretofore or contemporaneously
furnished by or on behalf of the Guarantor in writing to the Agent or any Lender
for purposes of or in connection with this Guaranty or any transaction
contemplated hereby, and all other such factual information hereafter furnished
by or on behalf of the Guarantor to the Agent, any LC Bank or any Lender will
be, true and accurate in every material respect on the date as of which such
information is dated or certified, and not incomplete by omitting to state any
material fact necessary to make such information not misleading.

     (n)  No Conditions Precedent. There are no conditions precedent to the
effectiveness of this Guaranty that have not been satisfied or waived.

     (o)  Reliance. The Guarantor has, independently and without reliance upon
the Borrower and based on such documents and information as it has deemed
appropriate, made its own credit analysis and decision to enter into this
Guaranty.

     (p)  Year 2000. The Guarantor has made a full and complete assessment of
the Year 2000 Issues and has a realistic and achievable Year 2000 Program. Based
on such assessment and on the Year 2000 Program, the Guarantor does not
reasonably anticipate that Year 2000 Issues will have a Material Adverse Effect.

     SECTION 7. Affirmative Covenants. The Guarantor covenants and agrees that,
so long as any part of the Obligations shall remain unpaid or any Lender shall
have any Commitment, the Guarantor will:

     (a)  Reporting Requirements.  Furnish to each Lender:

          (i)  as soon as available and in any event within 60 days after the
     end of each of the first three quarters of each fiscal year of the
     Guarantor, a consolidated balance sheet of the Guarantor and its
     Consolidated Subsidiaries as of the end of such quarter and consolidated
     statements of income, retained earnings and cash flows of the Guarantor and
<PAGE>

     its Consolidated Subsidiaries for the period commencing at the end of the
     previous fiscal year and ending with the end of such quarter, all in
     reasonable detail and duly certified (subject to year-end audit
     adjustments) by the chief financial officer or the Treasurer of the
     Guarantor as having been prepared in accordance (in all material respects)
     with GAAP consistently applied, except for (A) the absence of notes thereto
     and (B) changes in accounting principles required by GAAP; provided, that
     delivery of a copy of the Guarantor's Quarterly Report on Form 10-Q for
     such quarter shall be deemed to satisfy the foregoing requirement;

          (ii)  as soon as available and in any event within 120 days after the
     end of each fiscal year of the Guarantor and its Consolidated Subsidiaries,
     a copy of the Annual Report for such year for the Guarantor and its
     Consolidated Subsidiaries, containing a consolidated balance sheet of the
     Guarantor and its Consolidated Subsidiaries as at the end of such fiscal
     year and consolidated statements of income, retained earnings and cash
     flows of the Guarantor and its Consolidated Subsidiaries for such fiscal
     year, certified in a manner acceptable to the Agent by Arthur Andersen LLP
     or another nationally-recognized independent public accounting firm
     selected by the Guarantor and acceptable to the Agent; provided, that
     delivery of a copy of the Guarantor's Annual Report on Form 10-K
     (containing such statements) or Current Report on Form 8-K (containing such
     statements) for such year shall be deemed to satisfy the foregoing
     requirement;

          (iii)  concurrently with the financial statements for each quarterly
     accounting period and for each fiscal year of the Guarantor furnished
     pursuant to paragraphs (i) and (ii), above, (A) a certificate of the chief
     financial officer, any vice president responsible for financial or
     accounting matters, or the Treasurer of the Guarantor stating that (1) the
     Guarantor has performed and observed all of, and the Guarantor is not in
     default in the performance or observance of any of, the terms, covenants,
     agreements and conditions of this Guaranty or, if the Guarantor shall be in
     default, specifying all such defaults and the nature thereof, of which the
     signer of such certificate may have knowledge, and (2) the signer has
     obtained no knowledge of any Unmatured Default or Event of Default except
     as specified in such certificate, and (B) an analysis prepared and
     certified by the chief financial officer, any vice president responsible
     for financial or accounting matters, or the Treasurer of the Guarantor of
     the covenants contained in Sections 7(i) and (j), containing all
     information necessary for determining compliance by the Guarantor with such
     covenants;

          (iv)  as soon as available and in any event within 120 days after the
     end of each fiscal year of the Guarantor and concurrently with the
     financial statements furnished pursuant to paragraph (ii), above, a written
     statement of the independent public accountants that certified such
     financial statements stating that, in making the examination necessary for
     their certification of such financial statements, they have obtained no
     knowledge of any default by the Guarantor in the observance of any of the
     covenants contained in Section 7(i) or (j) or, if such accountants shall
     have obtained knowledge of any such default, specifying all such defaults
     and the nature thereof, it being understood that they shall not be liable
     directly or indirectly for any failure to obtain knowledge of any default;
<PAGE>

          (v)  as soon as possible and in any event within five days after the
     Guarantor becomes aware of the commencement of litigation against the
     Guarantor, the Borrower, or any of their respective Subsidiaries that could
     reasonably be expected to have a Material Adverse Effect, or that questions
     the validity or enforceability of any of the Loan Documents against the
     Guarantor or the Borrower, notice of such litigation describing in
     reasonable detail the facts and circumstances concerning such litigation
     and the Guarantor's, the Borrower's or such Subsidiary's, as the case may
     be, proposed actions in connection therewith; provided, that delivery of a
     copy of the Guarantor's Current Report on Form 8-K describing any such
     litigation shall be deemed to satisfy such requirement unless the Agent, or
     any Lender acting through the Agent, shall request any additional
     information relating to such litigation;

          (vi)  promptly after the sending or filing thereof, copies of all
     reports which the Guarantor sends to any of its security holders, and
     copies of all reports and registration statements (other than registration
     statements relating to (A) the offering of indebtedness or
     preferred/preference stock equity securities and (B) employee benefit
     plans) which the Guarantor or any of its Subsidiaries files with the
     Securities and Exchange Commission or any national securities exchange;

          (vii)  as soon as possible and in any event (A) within 30 days after
     any ERISA Event described in clause (i) of the definition of ERISA Event
     with respect to any Plan of the Guarantor or any ERISA Affiliate of the
     Guarantor has occurred and (B) within 10 days after any other ERISA Event
     with respect to any Plan of the Guarantor or any ERISA Affiliate of the
     Guarantor has occurred, a statement of the chief financial officer, any
     vice president responsible for financial or accounting matters, or the
     Treasurer of the Guarantor describing such ERISA Event and the action, if
     any, which the Guarantor or such ERISA Affiliate proposes to take with
     respect thereto;

          (viii)  promptly after receipt thereof by the Guarantor or any of its
     ERISA Affiliates from the PBGC copies of each notice received by the
     Guarantor or such ERISA Affiliate of the PBGC's intention to terminate any
     Plan of the Guarantor or such ERISA Affiliate or to have a trustee
     appointed to administer any such Plan;

          (ix)  promptly after receipt thereof by the Guarantor or any ERISA
     Affiliate of the Guarantor from a Multiemployer Plan sponsor, a copy of
     each notice received by the Guarantor or such ERISA Affiliate concerning
     the imposition or amount of withdrawal liability in an aggregate principal
     amount of at least $25,000,000 pursuant to Section 4202 of ERISA in respect
     of which the Guarantor or such ERISA Affiliate is reasonably expected to be
     liable;

          (x)  as soon as possible and in any event within ten days after the
     Guarantor knows or should have reason to know of the occurrence of each
     Unmatured Default or Event of Default continuing on the date of such
     statement, a statement of the chief financial officer, any vice president
     responsible for financial or accounting matters, or the Treasurer of the
     Guarantor setting forth details of such Unmatured Default or Event of
     Default and the action that the Guarantor or the Borrower has taken and
     proposes to take with respect thereto; and
<PAGE>

          (xi)  such other information (other than proprietary customer
     information) respecting the business, assets, revenues, financial
     condition, results of operations, operations or prospects of the Guarantor,
     the Borrower, or any of their respective Subsidiaries as the Agent or any
     Lender may from time to time reasonably request.

     (b)  Preservation of Corporate Existence, Etc. Preserve and maintain, and
cause each of its Subsidiaries to preserve and maintain, its legal existence in
the jurisdiction of its organization and qualify and remain qualified as a
foreign organization in each jurisdiction in which such qualification is
reasonably necessary in view of its business and operations or the ownership of
its properties, and preserve, renew and keep in full force and effect the
rights, privileges and franchises necessary or desirable in the normal conduct
of its business; provided however, that neither the Guarantor nor any such
Subsidiary shall be required to preserve and maintain any such right, privilege
or franchise, and no Subsidiary shall be required to preserve and maintain its
corporate existence, unless the failure to do so would have a Material Adverse
Effect.

     (c)  Compliance with Laws, Etc. Comply, and cause each of its Subsidiaries
to in all material respects with all Applicable Laws, such compliance to include
compliance with ERISA and Environmental Laws.

     (d)  Maintenance of Insurance, Etc. Maintain, and cause each of its
Subsidiaries to maintain, such insurance as may be required by law and such
other insurance, to the extent and against such hazards and liabilities, as is
customarily maintained by companies similarly situated.

     (e)  Inspection Rights. At any reasonable time and from time to time as the
Agent or any Lender may reasonably request, permit the Agent, each Lender or any
agents or representatives thereof to examine and make copies of and abstracts
from the records and books of account of, and visit the properties of, the
Guarantor and any of its Subsidiaries (except in the case of Commonwealth, as
may be restricted by law), and to discuss the affairs, finances and accounts of
the Guarantor and any of its Subsidiaries with any of their respective officers
or directors; provided, however, that, prior to the disclosure of any
information or materials of the Guarantor or its Subsidiaries relating to
customers, pricing methods or formulae or proprietary methods or processes, the
Guarantor may require the Lender seeking to inspect the same to enter into a
confidentiality and nondisclosure agreement with respect to the use and
disclosure of such information or materials in form and substance reasonably
satisfactory to the Guarantor and such Lender and containing customary terms.

     (f)  Maintaining of Books. Maintain, and cause each of its Subsidiaries to
maintain, complete and accurate books of record and account in which entries
shall be made of all financial transactions and the assets and business of the
Guarantor and each of its Subsidiaries in accordance with GAAP.

     (g)  Maintenance of Properties. Cause all properties used or useful in the
conduct of the business of the Guarantor or any of its Subsidiaries to be
maintained and kept in reasonable condition, repair and working order, and cause
to be made all necessary repairs, renewals, replacements, betterments and
improvements thereof, all as in the judgment of the Guarantor may be necessary
so that the business carried on in connection therewith may be properly and
<PAGE>

advantageously conducted at all times; except where the failure to do so would
not have a Material Adverse Effect.

     (h)  Taxes and Liabilities. Pay, and cause each of its Subsidiaries to pay,
when due all taxes, assessments, governmental charges and other liabilities
imposed upon it or its property, except to the extent contested in good faith
and by appropriate proceedings and in respect of which adequate reserves for the
payment thereof have been set aside by the Guarantor or such Subsidiary, as the
case may be, in accordance with GAAP.

     (i)  Maintenance of Minimum Consolidated Tangible Net Worth. Maintain at
all times a Consolidated Tangible Net Worth of at least $3,500,000,000.

     (j)  Consolidated Leverage Ratio. Maintain, on the last day of each fiscal
quarter, a ratio of (i) Consolidated Indebtedness to (ii) Consolidated
Capitalization of not greater than 0.65 to 1.

     (k)  Ownership of Subsidiaries. Maintain direct ownership of 100% of the
capital stock of the Borrower and maintain direct ownership of at least 80% of
the voting capital stock of Commonwealth (in either case, a "Change in
Control"); provided, however, that the proposed Unicom/PECO Merger shall not
constitute a Change of Control.

     (l)  Year 2000. Take all such actions as is reasonably necessary to
successfully implement the Year 2000 Program and to assure that Year 2000 Issues
will not have a Material Adverse Effect. At the request of the Agent, the
Guarantor will provide a description of the Year 2000 Program, together with any
updates or progress reports with respect thereto.

     SECTION 8. Negative Covenants. The Guarantor covenants and agrees that, so
long as any part of the Obligations shall remain unpaid or any Lender shall have
any Commitment, the Guarantor will not:

     (a)  Liens, Etc. Create or suffer to exist, or permit any of its
Subsidiaries to create or suffer to exist, any lien, security interest, or other
charge or encumbrance, or any other type of preferential arrangement, upon or
with respect to any of its properties (including, without limitation, the
capital stock of or any other equity interest in any of its Subsidiaries, except
as provided below), whether now owned or hereafter acquired, or assign, or
permit any of its Subsidiaries to assign, any right to receive income, in each
case to secure or provide for the payment of any Indebtedness of any Person (any
of the foregoing being referred to herein as a "Lien"), other than (i) Liens
imposed by law, such as carriers', warehousemen's and mechanics' Liens and other
similar Liens arising in the ordinary course of business; (ii) with respect to
any Subsidiary of the Guarantor, other Liens of the types described in Section
6.16 of the Credit Agreement; (iii) Liens on the capital stock of or any other
equity interest in any of the Guarantor's Subsidiaries or any such Subsidiary's
assets to secure Nonrecourse Indebtedness, (iv) Liens created in connection with
the acquisition by Subsidiaries of assets and the continuation of such Liens in
connection with any refinancing of the Indebtedness secured by such Liens,
provided such Liens are limited to the assets so acquired; (v) Liens on the
assets and/or rights to receive income of any Person that exist at the time such
Person becomes a Subsidiary and the continuation of such Liens in connection
with any refinancing or restructuring of the obligations secured by such Liens;
and (vi) Liens arising under the Mortgage, dated July 1,
<PAGE>

1923, as amended and supplemented by supplemental indentures, including the
Supplemental Indenture, dated August 1, 1944, from Commonwealth to Harris Trust
and Savings Bank and D.G. Donovan, as trustees, and "permitted liens" as defined
in said Mortgage; provided however, that, notwithstanding the foregoing, if both
before and after giving effect thereto no Unmatured Default or Event of Default
shall have occurred and be continuing, Commonwealth may sell, pledge or
otherwise dispose of its accounts receivable.

     (b)  Compliance with ERISA.  (i) Permit to exist any "accumulated funding
deficiency" (as defined in Section 412(a) of the Internal Revenue Code of 1986,
as amended from time to time) (unless such deficiency exists with respect to a
Multiple Employer Plan or Multiemployer Plan and the Guarantor has no control
over the reduction or elimination of such deficiency), (ii) terminate, or permit
any ERISA Affiliate of the Guarantor to terminate, any Plan of the Guarantor or
such ERISA Affiliate so as to result in a liability of $25,000,000 or more of
the Guarantor to the PBGC, or (iii) permit to exist any occurrence of any
Reportable Event (as defined in Title IV of ERISA), other than a Reportable
Event for which the 30-day notice requirement with respect thereto has been
waived by the PBGC or any other event or condition, which presents a material
(in the reasonable opinion of the Required Lenders) risk of such a termination
by the PBGC of any Plan of the Guarantor or such ERISA Affiliate and such a
liability to the Guarantor.

     (c)  Mergers, Etc.  Merge or consolidate with or into any Person, or
convey, transfer, lease or otherwise dispose of (whether in one transaction or
in a series of related or unrelated transactions, and whether in a
sale/leaseback transaction or otherwise) more than 10% of its assets (whether
now owned or hereafter acquired), unless, in the case of a merger, immediately
after giving effect thereto, (i) no event shall occur and be continuing that
constitutes an Unmatured Default or an Event of Default, (ii) the Guarantor is
the surviving corporation, (iii) the Guarantor's Consolidated Tangible Net Worth
shall be equal to or greater than its Consolidated Tangible Net Worth
immediately prior to such merger and (iv) the Guarantor shall not be liable with
respect to any Indebtedness or allow its property to be subject to any Lien
which it could not become liable with respect to or allow its property to become
subject to under this Guaranty on the date of such transaction, provided,
however, nothing in this Section shall prohibit the proposed Unicom/PECO Merger.

     (d)  Indebtedness.  Create, incur, assume or suffer to exist any
Indebtedness, other than (without duplication) (i) Indebtedness to the Borrower
in an amount not to exceed $25,000,000 in the aggregate at any one time
outstanding, (ii) Indebtedness hereunder, (iii) unsecured Contingent Obligations
(other than in respect of this Guaranty) in an aggregate amount at any one time
outstanding not to exceed the excess of (A) $700,000,000 over (B) the amount of
Contingent Obligations incurred by the Borrower pursuant to Section 6.13(ii) of
the Credit Agreement, (iv) other unsecured Indebtedness and (v) secured
Indebtedness in connection with investments in partnership or limited liability
company entities that invest, directly or indirectly, in residential apartment
complexes qualifying for low income housing tax credits under Section 42 of the
Code in an aggregate amount not to exceed $75,000,000; provided, however, that,
notwithstanding the foregoing, the aggregate amount of Indebtedness of the
Guarantor, the Borrower and Subsidiaries of the Borrower at any one time
outstanding shall not exceed $900,000,000, provided, further, that for purposes
of this Section the term "Indebtedness" shall not include the Unicom Investment
Inc. Debt.

<PAGE>

     (e)  Guarantor and Subsidiaries' Stock.  Permit any of its  Subsidiaries to
purchase or otherwise acquire any shares of capital stock of the Guarantor; or
take any action, or permit any such Subsidiary to take any action, that would
result in a material decrease in the percentage of the outstanding shares of
capital stock of any "Significant Subsidiary" of the Guarantor (within the
meaning of Rule 1-02 of the Regulation S-X of the Securities and Exchange
Commission) owned by the Guarantor and its other Subsidiaries; provided,
however, that the Guarantor or Commonwealth may take any such action with
respect to the capital stock of Commonwealth, provided that, after giving effect
to any such action, the Guarantor is in compliance with Section 7(k) hereof.

     (f)  Other Agreements.  Enter into any agreement containing any provision
that would be violated or breached by the performance of its obligations
hereunder or under any instrument or document delivered or to be delivered by
the Guarantor hereunder or in connection herewith.

     (g)  Transactions with Affiliates.  Enter into, or permit any of its
Subsidiaries to enter into, any transaction with an Affiliate of the Guarantor,
unless (i) such transaction is on terms no less favorable to the Guarantor or
such Subsidiary, as the case may be, than if the transaction had been negotiated
in good faith on an arm's length basis with a Person that was not an Affiliate
of the Guarantor or (ii) such transaction is conducted pursuant to the
Affiliated Interests Agreement, dated as of December 4, 1995, among
Commonwealth, the Guarantor and the other entities named therein, as it may be
amended or modified from time to time or replaced by an agreement regarding such
transactions approved by the Illinois Commerce Commission or the Securities and
Exchange Commission; provided, the foregoing shall not apply to (x) the
transactions contemplated by the Asset Sale Agreement, dated as of May 11, 1999,
between Commonwealth and Unicom Investment Inc. relating to the sale of
Commonwealth's fossil generation assets and the incurrence of the Unicom
Investment Inc. Debt; (y) the transfer by Commonwealth to Unicom Technology
Development Inc., an Illinois corporation, of up to $275,000,000 of the notes
representing the Unicom Investment Inc. Debt under said Asset Sale Agreement
along with an obligation in respect of an equivalent amount of Commonwealth's
contingent obligation to pay post-retirement health care benefits; and (z)
transactions associated with a transfer of Commonwealth's nuclear generating
stations to a Subsidiary of the Guarantor.

     (h)  Distributions.  Upon the occurrence and during the continuance of an
Event of Default, declare or pay, directly or indirectly, any dividend, payment
or other distribution of assets, properties, cash, rights, obligations or
securities on account of any share of any class of capital stock of the
Guarantor, or purchase, redeem, retire, or otherwise acquire for value, or
permit any of its Subsidiaries to purchase, redeem, retire, or otherwise acquire
for value, any shares of any class of capital stock of the Guarantor or any
warrants, rights, or options to acquire any such shares, now or hereafter
outstanding, or make any distribution of assets to any of its shareholders;
provided, however, that, notwithstanding the foregoing, the Guarantor may, to
the extent that it is legally required to do so, pay any such dividend, payment
or other distribution after the Guarantor has declared such dividend, payment or
other distribution.

     SECTION 9.  Amendments, Etc.  No amendment or waiver of any provision of
this Guaranty, and no consent to any departure by the Guarantor herefrom, shall
in any event be effective unless the same shall be in writing and signed by the
Agent and the Required Lenders, and then such waiver or consent shall be
effective only in the specific instance and for the

<PAGE>

specific purpose for which given, provided, however, that no amendment, waiver
or consent shall, unless in writing and signed by all of the Lenders, (i) limit
the liability of, or release, the Guarantor hereunder, (ii) postpone any date
fixed for payment hereunder, or (iii) change the number of Lenders required to
take any action hereunder.

     SECTION 10.  Addresses for Notices.  All notices and other communications
provided for hereunder shall be in writing (including telecopier, telegraphic or
cable communication) and mailed, telecopied, telegraphed, cabled or delivered to
it, (i) if to the Guarantor, at its address at P.O. Box A-3005, 10 South
Dearborn Street, 37th Floor, Chicago, Illinois 60690-3005, Attention: Treasurer,
Telecopy: (312) 394-4082, with a copy of the same address, attention: Associate
General Counsel-Corporate and Commercial, telecopy: (312) 394-3950, and (ii) if
to the Agent, the LC Issuer or any Lender, at its address specified in the
Credit Agreement or, as to any party, at such other address as shall be
designated by such party in a written notice to each other party. All such
notices and other communications shall, when mailed, telecopied, telegraphed or
cabled, be effective when deposited in the mails, telecopied, delivered to the
telegraph company or delivered to the cable company, respectively.

     SECTION 11.  No Waiver; Remedies.  No failure on the part of the Agent, the
LC Issuer or any Lender to exercise, and no delay in exercising, any right
hereunder shall operate as a waiver thereof; nor shall any single or partial
exercise of any right hereunder preclude any other or further exercise thereof
or the exercise of any other right. The remedies herein provided are cumulative
and not exclusive of any remedies provided by law.

     SECTION 12.  Right of Set-off.  Upon the occurrence and during the
continuance of any Default, the LC Issuer and each Lender is hereby authorized
at any time and from time to time, to the fullest extent permitted by law, to
set off and apply any and all deposits of the Guarantor (general or special,
time or demand, provisional or final). The LC Issuer and each Lender agrees
promptly to notify the Agent and the Guarantor after any such set-off and
application made by such Lender, provided that the failure to give such notice
shall not affect the validity of such set-off and application. The rights of the
LC Issuer and each Lender under this Section are in addition to other rights and
remedies (including, without limitation, other rights of set-off) that the LC
Issuer or such Lender may have.

     SECTION 13.  Continuing Guaranty; Assignments under Credit Agreement.  This
Guaranty is a continuing guaranty and shall (i) subject to the last sentence of
Section 3, remain in full force and effect until the later to occur of (A) the
payment in full of the Obligations and all other amounts payable under this
Guaranty and (B) the expiration or termination of the Commitments, (ii) be
binding upon the Guarantor, its successors and assigns (provided, that the
Guarantor may not assign any of its rights or obligations hereunder without the
prior written consent of the Lenders), and (iii) inure to the benefit of, and be
enforceable by, the Agent, the LC Issuer, the Lenders and their respective
successors, transferees and assigns.  Without limiting the generality of the
foregoing clause (iii), any Lender may assign or otherwise transfer all or any
portion of its rights and obligations under the Credit Agreement (including,
without limitation, all or any portion of its Commitment, the Advances owing to
it and any Note held by it) to any other person or entity pursuant to Section
12.03 thereof, and such other person or entity shall thereupon become vested
with all the benefits in respect thereof granted to such Lender herein or

<PAGE>

otherwise, subject, however, to the provisions of Article X (concerning the
Agent) of the Credit Agreement.

     SECTION 14.  Governing Law.  THIS GUARANTY SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

     SECTION 15.  Consent to Jurisdiction; Waiver of Jury Trial.  (a)  The
Guarantor hereby irrevocably (i) submits to the non-exclusive jurisdiction of
any New York State or Federal court sitting in New York City and any appellate
court from any thereof in any action or proceeding arising out of or relating to
this Guaranty or any other Loan Document and (ii) agrees that all claims in
respect of such action or proceeding may be heard and determined in such New
York State court or in such Federal court.  The Guarantor hereby irrevocably
waives the defense of an inconvenient forum to the maintenance of such action or
proceeding and any objection to venue in connection therewith.  The Guarantor
also irrevocably consents to the service of any and all process in any such
action or proceeding by the mailing by certified mail of copies of such process
to the Guarantor at its address specified in Section 10.  The Guarantor agrees
that a final judgment in any such action or proceeding shall be conclusive and
may be enforced in other jurisdictions by suit on the judgment or in any other
manner provided by law.

     (b)  THE GUARANTOR HEREBY WAIVES ALL RIGHT TO TRIAL BY JURY IN ANY ACTION,
PROCEEDING OR COUNTERCLAIM ARISING OUT OF OR RELATING TO THIS GUARANTY, ANY
OTHER LOAN DOCUMENT, OR ANY OTHER INSTRUMENT OR DOCUMENT DELIVERED HEREUNDER OR
THEREUNDER.

     SECTION 16.  Execution in Counterparts.   This Guaranty may be executed in
any number of counterparts and by different parties hereto in separate
counterparts, each of which counterparts when so executed shall be deemed to be
an original and all of which taken together shall constitute one and the same
agreement.

     SECTION 17.  Severability.  Any provision of this Guaranty or any other
Loan Document that is prohibited, unenforceable or invalid in any jurisdiction
shall, as to such jurisdiction, be ineffective to the extent of such
prohibition, unenforceability or invalidity without invalidating the remaining
provisions hereof or thereof or affecting the validity, enforceability or
legality of such provision in any other jurisdiction.

     SECTION 18.  Headings.  Article and Section headings used herein are for
convenience of reference only, are not part of this Guaranty and are not to
affect the construction of, or to be taken into consideration in interpreting,
this Guaranty.

     SECTION 19.  Entire Agreement.  This Guaranty constitutes the entire
agreement and understanding among the Guarantor, the Lenders, the LC Issuer and
the Agent relative to the subject matter hereof.  Any previous agreement by or
among such parties with respect to the subject matter hereof is superseded by
this Guaranty.  Nothing in this Guaranty, expressed or implied, is intended to
confer upon any party other than the Lenders, the LC Issuer and the Agent any
rights, remedies, obligations, or liabilities under or by reason of this
Guaranty.

<PAGE>

     IN WITNESS WHEREOF, the Guarantor has caused this Guaranty to be duly
executed and delivered by its officer thereunto duly authorized as of the date
first above written.

                                       UNICOM CORPORATION



                                       By /s/ Patricia L. Kampling
                                         -----------------------------------
                                          Patricia L. Kampling
                                          Treasurer