<PAGE>
 
                                         Exhibit (10)-21
                                         Unicom Corporation and
                                         Commonwealth Edison Company
                                         Form 10-K File Nos. 1-11375 and 1-1839

                                       


                    THIRD AMENDMENT TO EMPLOYMENT AGREEMENT


        This THIRD AMENDMENT dated as of March 8th, 1999 (the "Third Amendment")
to the Employment Agreement dated as of March 10, 1998 by and among the parties
hereto, as previously amended by the First and Second Amendments thereto (as so
amended, the "Existing Employment Agreement"), is made by and among Unicom
Corporation, an Illinois corporation ("Unicom"), Commonwealth Edison Company, an
Illinois corporation ("ComEd"), and John W. Rowe ("Executive").

        WHEREAS, the Compensation Committee of the Board of Directors of each of
Unicom and ComEd and Executive have determined that it is desirable to amend and
supplement certain provisions of the Existing Employment Agreement relating to a
potential Change in Control of ComEd or Unicom and to clarify certain other
provisions of the Existing Employment Agreement; and

        NOW THEREFORE, in consideration of their mutual undertakings set forth
in this Third Amendment, the parties hereto agree as follows:


                                       I

    Each term used in this Third Amendment which is defined in the Existing
Employment Agreement shall have the meaning specified in therein unless
otherwise defined in this Third Amendment.  Each reference to "this Agreement"
in the Existing Employment Agreement shall, from and after the date hereof,
refer to the Existing Employment Agreement as amended by this Third Amendment.


                                      II

    Section 1.9 is amended by substituting "Article IX" for "Article VIII."


                                      III

    Section 1.39 is amended by substituting "the first 24 months" for the words
"two years" immediately preceding "after a Change in Control" in the proviso at
the end of such Section.

                                      -1-
<PAGE>
 
                                      IV

    Section 1.41 is amended and restated to read in its entirety as follows:


            1.41  "Taxes" means federal, state, local or other income,
                   -----                                              
        employment or other taxes.


                                       V

    Sections 6.3(a) and 6.3(b) are amended and restated to read in their
entirety as follows:

            6.3 Retirement Benefits.
                ------------------- 

            (a) Upon the first to occur of Executive's Early Retirement or
        Normal Retirement, a Termination Without Cause, a Termination for Good
        Reason, or a Termination of Employment by reason of death or Disability
        (any of the foregoing, a "SERP Payment Event"), Executive (or, in the
        event of his death, his surviving spouse) shall thereafter receive a
        retirement benefit (the "SERP Benefit") determined pursuant to Section
        6.3(b).

            (b) The SERP Benefit to be provided to Executive during any year
        shall equal an amount which, when added to all other retirement benefits
        provided to Executive by the Company and its Affiliates during such year
        (including payments under the Service Annuity System, the Supplemental
        Retirement Plan, the Social Security supplement paid by ComEd until
        Executive attains age 65, any retirement benefit paid pursuant to
        Section 8.3(c), and any other sources) results an aggregate annual
        retirement benefit equal to the annual retirement benefit that would
        have been payable under the Service Annuity System (including under the
        Supplemental Retirement Plan) as in effect on the Agreement Date,
        calculated as though Executive had:

                (i)  retired at age 60 (or, if greater, his attained age upon
            the first SERP Payment Event), and

                (ii) accrued 20 years of service on the Commencement Date and
            one additional year of service on each Anniversary Date occurring on
            or before the Termination Date;

        provided, however, that in no event shall any SERP Benefit be payable:

                                      -2-
<PAGE>
 
                (A) during the Severance Period,

                (B) in the event that, before the first SERP Payment Event,
            Executive shall have received a Notice of Termination and his
            employment is subsequently terminated by the Company for Cause, or

                (C) in the event of any Termination of Employment other than in
            connection with a SERP Payment Event.

        In addition, Executive's right to receive the SERP Benefit shall be
        subject to Section 7.7.


                                      VI

     Section 7.3 is amended by revising the introductory phrase before paragraph
(a) of such Section to read in full as follows:

        Except as otherwise provided in Section 8.3, in the event of a
        Termination Without Cause or a Termination for Good Reason:


                                      VII

     Section 7.7 is amended by substituting "Article IX" for all references to
"Article VIII" included therein.


                                     VIII

     Article IX is redesignated as Article X; Article VIII is redesignated as
Article IX, and a new Article VIII is added to read in full as follows:

                                  ARTICLE VIII
                       EFFECTS OF CERTAIN CONTROL CHANGES

            8.1 Effect on Certain Defined Terms.  In the event of a Termination
                -------------------------------                                
        Without Cause or a Termination for Good Reason for which the Termination
        Date occurs at any time during the period beginning on the date on which
        a Change in Control first occurs and ending 24 months after such date
        (such period, the "Post-Change Period"):

(a)  the term "Formula Annual Incentive" shall mean the greater of (i)
               ------------------------                               
            that amount determined pursuant to Section 1.23 or (ii) Executive's
            target Annual Incentive determined as of the Termination Date; and

                                      -3-
<PAGE>
 
                (b) the term "Good Reason" shall have the meaning specified in
                              -----------                                     
            Section 1.24, except that, subject to the proviso at the end of
            Section 1.24, any one or more of the following acts or omissions
            shall also constitute Good Reason:

(i)  a determination by Executive, made in good faith at any time during the
                Post-Change Period, that, as a result of a Change in Control, he
                is substantially unable to perform, or that there has been a
                material reduction in, any of his duties, functions,
                responsibilities or authority;

                    (ii)  the failure for any reason of any successor to the
                Company to assume this Agreement in writing as required by
                Section 8.2;

                    (iii) a relocation of the principal offices of the Company
                at any time during the Post-Change Period by more than 50 miles
                from the location of such offices immediately before the date on
                which such Post-Change Period begins; or

                    (iv)  during any 12-month period commencing after the Change
                of Control, any increase of at least 20% in the amount of time
                that Executive is required to devote to business-related travel
                outside of the metropolitan Chicago, Illinois area relative to
                the amount of time that Executive devoted to such business
                travel during the 12-month period immediately prior to the
                Change in Control, but only to the extent that such increase is
                attributable to requirements imposed upon Executive by the
                Board.

            8.2 Successor(s).  Before the consummation of any Change in Control,
                ------------                                                    
        the Company shall obtain from each Person that becomes a successor of
        the Company by reason of the Change in Control, the unconditional
        written agreement of such Person to assume this Agreement and to perform
        all of the obligations of the Company hereunder.

            8.3 Termination Without Cause or for Good Reason During Post-Change
                ---------------------------------------------------------------
        Period.  In the event of a Termination Without Cause or a Termination
        ------                                                               
        for Good Reason for which the Termination Date occurs during a Post-
        Change Period, the provisions of Section 7.3 shall be inapplicable and,
        in lieu thereof:

                (a) Executive shall receive a lump sum equal to his Accrued Base
            Salary, Accrued Annual Incentive, and Prorated 

                                      -4-
<PAGE>
 
            Annual Incentive (determined by reference to the Formula Annual
            Incentive computed in accordance with Section 8.1(a));

                (b) Executive shall receive a lump sum equal to three (3.0)
            times the sum of (x) his Base Salary in effect during the calendar
            year preceding the Termination Date and (y) his Formula Annual
            Incentive (computed in accordance with Section 8.1(a)) determined as
            of the Termination Date;

                (c) Executive and his family shall receive for the duration of
            the Severance Period, a continuation of the benefits described in
            Section 6.4 to which Executive and his family are entitled as of the
            Termination Date (or, if such benefits are not available, the
            economic equivalent thereof) and, upon the expiration of the
            Severance Period, Executive and his spouse shall be entitled to
            Post-Retirement Health Care Coverage in accordance with the
            provisions of Section 7.6;

                (d) ComEd shall, at its expense, engage a professional
            outplacement organization which shall provide individual
            outplacement services to Executive for a period of six months
            commencing on the Termination Date, subject to extension for an
            additional period of six months in the sole discretion of ComEd;

                (e) each of Executive's Options that is exercisable on the
            Termination Date shall remain exercisable until the applicable
            Option Expiration Date;

                (f) each of Executive's Options that is not fully exercisable as
            of the Termination Date shall immediately become fully exercisable
            and shall thereafter remain exercisable until the applicable Option
            Expiration Date;

                (g) all forfeiture conditions which as of the Termination Date
            are applicable to any deferred stock unit, restricted stock or
            restricted share units awarded to Executive by the Company pursuant
            to the LTIP, a successor plan, or otherwise at any time during the
            Contract Term shall lapse immediately; and

(h)  If all or any portion of any of Executive's awards under any other bonus
            or incentive arrangement under the LTIP shall for any reason
            be unvested as of the Termination Date, the Company shall pay
            Executive a benefit equal to the increase in the benefit that
            Executive would have received if the unvested portion of such
            benefit had become fully vested as of the Termination Date.

                                      -5-
<PAGE>
 
            8.4 Enhanced Retirement Benefit.
                --------------------------- 

                (a) In the event of a Termination Without Cause or a Termination
        for Good Reason for which the Termination Date occurs during the Post-
        Change Period, the aggregate amount of Executive's annual retirement
        benefit pursuant to Section 6.3(a) shall be computed on the basis of the
        assumptions set forth in such Section 6.3(a), together with the
        additional assumptions that Executive had:

                (x) attained as of the Termination Date an age that is three
            greater than the age determined pursuant to clause (x) of Section
            6.3(a),

                (y) accrued a number of years of service that is three years
            greater than the number of years of service determined pursuant to
            clause (y) of Section 6.3(a), and

                (z) received the lump-sum severance benefit specified in Section
            8.3(b) in equal monthly installments during the Severance Period.

                (b) For purposes of applying the adjustments necessary to give
        effect to the form in which Executive may from time to time elect to
        receive his SERP Benefit pursuant to Section 6.3(c), the term "Service
        Annuity System" shall refer to Service Annuity System as in effect on
        the last date preceding the Post-Change Period if the amount of the SERP
        Benefit (in the form in which Executive elects to receive it) would
        otherwise be reduced by application of the adjustments provided for
        under the Service Annuity System as in effect as of the Termination
        Date.

            8.5 Gross-Up for Certain Taxes.
                -------------------------- 

                (a) If it is determined by the Company's independent auditors
        that any monetary or other benefit received or deemed received by
        Executive from the Company or any Affiliate pursuant to this Agreement
        or otherwise, whether or not in connection with a Change in Control
        (such monetary or other benefits collectively, the "Potential Parachute
        Payments"), is or will become subject to any excise tax under Section
        4999 of the Code or any similar tax under any United States federal,
        state, local or other law (such excise tax and all such similar taxes
        collectively, "Excise Taxes"), then the Company shall, subject to
        Sections 8.10 and 8.11, within five business days after such
        determination, pay Executive an amount (the "Gross-Up Payment") equal to
        the product of:

                                      -6-
<PAGE>
 
                (i)  the amount of such Excise Taxes multiplied by

                (ii) the Gross-Up Multiple (as defined in Section 8.8).

        The Gross-Up Payment is intended to compensate Executive for all Excise
        Taxes payable by Executive with respect to Potential Parachute Payments
        and all Taxes or Excise Taxes payable by Executive with respect to the
        Gross-Up Payment.

                (b)  The determination of the Company's independent auditors
        described in Section 8.5(a), including the detailed calculations of the
        amounts of the Potential Parachute Payments, Excise Taxes and Gross-Up
        Payment and the assumptions relating thereto, shall be set forth in a
        written certificate of such auditors (the "Company Certificate")
        delivered to Executive. Executive or the Company may at any time request
        the preparation and delivery to Executive of a Company Certificate. The
        Company shall cause the Company Certificate to be delivered to Executive
        as soon as reasonably possible after such request.

            8.6 Determination by Executive.
                -------------------------- 

                (a) If (i) the Company shall fail to deliver a Company
        Certificate to Executive within 30 days after its receipt of his written
        request therefor, or (ii) at any time after Executive's receipt of a
        Company Certificate, Executive disputes either (x) the amount of the
        Gross-Up Payment set forth therein or (y) the determination set forth
        therein to the effect that no Gross-Up Payment is due (whether by reason
        of Section 8.11 or otherwise), then Executive may elect to require the
        Company to pay a Gross-Up Payment in the amount determined by Executive
        as set forth in an Executive Counsel Opinion (as defined in Section
        8.9).  Any such demand by Executive shall be made by delivery to the
        Company of a written notice which specifies the Gross-Up Payment
        determined by Executive (together with the detailed calculations of the
        amounts of Potential Parachute Payments, Excise Taxes and Gross-Up
        Payment and the assumptions relating thereto) and an Executive Counsel
        Opinion regarding such Gross-Up Payment (such written notice and opinion
        collectively, the "Executive's Determination").  Within 30 days after
        delivery of an Executive's Determination to the Company, the Company
        shall either (i) pay Executive the Gross-Up Payment set forth in the
        Executive's Determination (less the portion thereof, if any, previously
        paid to Executive by the Company) or (ii) deliver to Executive a Company
        Certificate and a Company Counsel Opinion (as defined in Section 8.9),
        and pay Executive the Gross-Up Payment specified in such Company

                                      -7-
<PAGE>
 
        Certificate.  If for any reason the Company fails to comply with the
        preceding sentence, the Gross-Up Payment specified in the Executive's
        Determination shall be controlling for all purposes.

                (b) If Executive does not request a Company Certificate, and the
        Company does not deliver a Company Certificate to Executive, then (i)
        the Company shall, for purposes of Section 8.11, be deemed to have
        determined that no Gross-Up Payment is due and (ii) Executive shall not
        pay any Excise Taxes in respect of Potential Parachute Payments except
        in accordance with Sections 8.10(a) or (d).

            8.7 Additional Gross-Up Amounts.  If for any reason (whether
                ---------------------------                             
        pursuant to subsequently enacted provisions of the Code, final
        regulations or published rulings of the IRS, a final judgment of a court
        of competent jurisdiction, a determination of the Company's independent
        auditors set forth in a Company Certificate or, subject to the last two
        sentences of Section 8.6(a), an Executive's Determination) it is later
        determined that the amount of Excise Taxes payable by Executive is
        greater than the amount determined by the Company or Executive pursuant
        to Section 8.5 or 8.6, as applicable, then the Company shall, subject to
        Sections 8.10 and 8.11, pay Executive an amount (which shall also be
        deemed a Gross-Up Payment) equal to the product of:

                (a) the sum of (1) such additional Excise Taxes and (2) any
            interest, penalties, expenses or other costs incurred by Executive
            as a result of having taken a position in accordance with a
            determination made pursuant to Section 8.5 or 8.6, as applicable,

        multiplied by

                (b) the Gross-Up Multiple.

            8.8 Gross-Up Multiple.  The Gross-Up Multiple shall equal a
                -----------------                                      
        fraction, the numerator of which is one (1.0), and the denominator of
        which is one (1.0) minus the lesser of (i) the sum, expressed as a
        decimal fraction, of the effective after-tax marginal rates of all Taxes
        and any Excise Taxes applicable to the Gross-Up Payment or (ii) 0.80, it
        being intended that the Gross-Up Multiple shall in no event exceed five
        (5.0).  (If different rates of tax are applicable to various portions of
        a Gross-Up Payment, the weighted average of such rates shall be used.)

            8.9 Opinion of Counsel.  "Executive Counsel Opinion" means an
                ------------------                                       
        opinion of nationally-recognized executive compensation counsel to the
        effect (i) that the amount of the Gross-Up Payment determined by
        Executive pursuant to Section 8.6 is the amount that a court of
        competent jurisdiction, based on a final judgment not subject to further
        appeal, is 

                                      -8-
<PAGE>
 
        most likely to decide to have been calculated in accordance with this
        Article and applicable law and (ii) if the Company has previously
        delivered a Company Certificate to Executive, that there is no
        reasonable basis or no substantial authority for the calculation of the
        Gross-Up Payment set forth in the Company Certificate. "Company Counsel
        Opinion" means an opinion of nationally-recognized executive
        compensation counsel to the effect that (i) the amount of the Gross-Up
        Payment set forth in the Company Certificate is the amount that a court
        of competent jurisdiction, based on a final judgment not subject to
        further appeal, is most likely to decide to have been calculated in
        accordance with this Article and applicable law and (ii) for purposes of
        Section 6662 of the Code, Executive has substantial authority to report
        on his federal income tax return the amount of Excise Taxes set forth in
        the Company Certificate.

            8.10 Amount Increased or Contested.
                 ----------------------------- 

                 (a) Executive shall notify the Company in writing (an
        "Executive's Notice") of any claim by the IRS or other taxing authority
        (an "IRS Claim") that, if successful, would require the payment by
        Executive of Excise Taxes in respect of Potential Parachute Payments in
        an amount in excess of the amount of such Excise Taxes determined in
        accordance with Section 8.5 or 8.6, as applicable.  Such Executive's
        Notice shall include the nature and amount of such IRS Claim, the date
        on which such IRS Claim is due to be paid (the "IRS Claim Deadline"),
        and a copy of all notices and other documents or correspondence received
        by Executive in respect of such IRS Claim.  Executive shall give his
        Executive's Notice as soon as practicable, but no later than the earlier
        of (i) 10 business days after Executive first obtains actual knowledge
        of such IRS Claim or (ii) five business days before the IRS Claim
        Deadline; provided, however, that Executive's failure to give such
        notice shall affect the Company's obligations under this Article only to
        the extent that the Company is actually prejudiced by such failure.  If
        at least one business day before the IRS Claim Deadline the Company
        shall:

                (1) deliver to Executive a Company Certificate to the effect
            that the IRS Claim has been reviewed by the Company's independent
            auditors and, notwithstanding the IRS Claim, the amount of Excise
            Taxes, interest and penalties payable by Executive is either zero or
            an amount less than the amount specified in the IRS Claim,

                (2) pay to Executive an amount (which shall also be deemed a
            Gross-Up Payment) equal to the positive difference between (x) the
            product of the amount of Excise Taxes, interest and penalties
            specified in the Company Certificate, if any, 

                                      -9-
<PAGE>
 
            multiplied by the Gross-Up Multiple, and (y) the portion of such
            product, if any, previously paid to Executive by the Company, and

                (3) direct Executive pursuant to Section 8.10(d) to contest the
            balance of the IRS Claim,

        then Executive shall pay only the amount, if any, of Excise Taxes,
        interest and penalties specified in the Company Certificate.  In no
        event shall Executive pay an IRS Claim earlier than 30 days after having
        given an Executive's Notice to the Company (or, if sooner, the IRS Claim
        Deadline).

                (b) At any time after the payment by Executive of any amount of
        Excise Taxes or related interest or penalties in respect of Potential
        Parachute Payments (whether or not such amount was based upon a Company
        Certificate, an Executive's Determination or an IRS Claim), the Company
        may in its discretion require Executive to pursue a claim for a refund
        (a "Refund Claim") of all or any portion of such Excise Taxes, interest
        or penalties as the Company may specify by written notice to Executive.

                (c) If the Company notifies Executive in writing that the
        Company desires Executive to contest an IRS Claim or to pursue a Refund
        Claim, Executive shall:

                (i)   give the Company all information that it reasonably
            requests in writing from time to time relating to such IRS Claim or
            Refund Claim, as applicable,

                (ii)  take such action in connection with such IRS Claim or
            Refund Claim (as applicable) as the Company reasonably requests in
            writing from time to time, including accepting legal representation
            with respect thereto by an attorney selected by the Company, subject
            to the approval of Executive (which approval shall not be
            unreasonably withheld or delayed),

                (iii) cooperate with the Company in good faith to contest such
            IRS Claim or pursue such Refund Claim, as applicable,

                (iv)  permit the Company to participate in any proceedings
            relating to such IRS Claim or Refund Claim, as applicable, and

                (iv)  contest such IRS Claim or prosecute such Refund Claim (as
            applicable) to a determination before any administrative tribunal,
            in a court of initial jurisdiction and in one or more appellate
            courts, as the Company may from time to time determine 

                                     -10-
<PAGE>
 
            in its discretion.

        The Company shall control all proceedings in connection with such IRS
        Claim or Refund Claim (as applicable) and in its discretion may cause
        Executive to pursue or forego any and all administrative appeals,
        proceedings, hearings and conferences with the IRS or other taxing
        authority in respect of such IRS Claim or Refund Claim (as applicable);
        provided that (i) any extension of the statute of limitations relating
        to payment of taxes for the taxable year of Executive relating to the
        IRS Claim is limited solely to such IRS Claim, (ii) the Company's
        control of the IRS Claim or Refund Claim (as applicable) shall be
        limited to issues with respect to which a Gross-Up Payment would be
        payable, and (iii) Executive shall be entitled to settle or contest, as
        the case may be, any other issue raised by the IRS or other taxing
        authority.

                (d) The Company may at any time in its discretion direct
        Executive to (i) contest the IRS Claim in any lawful manner or (ii) pay
        the amount specified in an IRS Claim and pursue a Refund Claim;
        provided, however, that if the Company directs Executive to pay an IRS
        Claim and pursue a Refund Claim, the Company shall advance the amount of
        such payment to Executive on an interest-free basis and shall indemnify
        Executive, on an after-tax basis, for any Taxes, Excise Taxes, and any
        related interest or penalties imposed with respect to such advance.

                (e) The Company shall pay directly all legal, accounting and
        other costs and expenses (including additional interest and penalties)
        incurred by the Company or Executive in connection with any IRS Claim or
        Refund Claim, as applicable, and shall indemnify Executive, on an after-
        tax basis, for any Taxes, Excise Taxes and related interest and
        penalties imposed on Executive as a result of such payment of costs and
        expenses.

            8.11  Limitation on Gross-Up Payments.
                  ------------------------------- 

(a)  Notwithstanding any other provision of this Article VIII, if the
        aggregate After-Tax Amount (as defined below) of the Potential Parachute
        Payments and Gross-Up Payment that, but for this Section 8.11, would be
        payable to Executive, does not exceed 110% of the After-Tax Floor Amount
        (as defined below), then no Gross-Up Payment shall be made to Executive
        and the aggregate amount of Potential Parachute Payments payable to
        Executive shall be reduced (but not below the Floor Amount) to the
        largest amount which would both (i) not cause any Excise Taxes to be
        payable by Executive and (ii) not cause any Potential Parachute Payments
        to become nondeductible by the Company by reason of Section 280G of the
        Code (or any successor provision).  For purposes of the preceding

                                     -11-
<PAGE>
 
        sentence, Executive shall be deemed to be subject to the highest
        effective after-tax marginal rate of Taxes.

                (b) For purposes of this Section:

                     (i) "After-Tax Amount" means the portion of a specified
                amount that would remain after payment of all Taxes and Excise
                Taxes paid or payable by Executive in respect of such specified
                amount;

                    (ii) "Floor Amount" means the greatest pre-tax amount of
                Potential Parachute Payments that could be paid to Executive
                without causing him to become liable for any Excise Taxes in
                connection therewith; and

                    (iii)  "After-Tax Floor Amount" means the After-Tax Amount
                            ----------------------                            
                of the Floor Amount.

            8.12  Refunds.  If, after the receipt by Executive of any payment or
                  -------                                                       
        advance of Excise Taxes by the Company pursuant to this Article,
        Executive receives any refund with respect to such Excise Taxes,
        Executive shall (subject to the Company's complying with any applicable
        requirements of Section 8.10) promptly pay the Company the amount of
        such refund (together with any interest paid or credited thereon after
        Taxes applicable thereto).  If, after the receipt by Executive of an
        amount advanced by the Company pursuant to Section 8.10, a determination
        is made that Executive shall not be entitled to any refund with respect
        to such claim and the Company does not notify Executive in writing of
        its intent to contest such determination within 30 days after the
        Company receives written notice of such determination, then such advance
        shall be forgiven and shall not be required to be repaid and the amount
        of such advance shall offset, to the extent thereof, the amount of
        Gross-Up Payment required to be paid.  Any contest of a denial of refund
        shall be controlled by Section 8.10.

            8.13  No Adverse Effect on Pooling of Interests.  Any benefits
                  -----------------------------------------               
        provided to Executive under this Article VIII may be reduced or
        eliminated to the extent necessary, in the reasonable judgment of the
        Unicom Board, to enable the Company to account for a merger,
        consolidation or similar transaction as a pooling of interests; provided
        that (i) the Unicom Board shall have exercised such judgment and given
        Executive written notice thereof prior to the Effective Date, (ii) the
        determination of the Unicom Board shall be supported by a written
        certificate of the Company's independent auditors, a copy of which shall
        be provided to the Executive before the Effective Date, and (iii) if for
        any reason the Company shall, notwithstanding such exercise of 

                                     -12-
<PAGE>
 
        judgment by the Unicom Board, fail to account for such
        merger, consolidation or other transaction as a pooling 
        of interests for any fiscal period that includes the 
        Termination Date, the Company shall, as soon as 
        practicable after the end of such fiscal period and, in 
        any event, within 10 business days after the its audited 
        financial statements for such fiscal period first become 
        publicly available, provide to Executive the excess, if 
        any, of (x) each severance or other benefit determined 
        pursuant to Article VIII (without giving effect to this 
        Section 8.13) over (y) each corresponding severance or 
        other benefit previously provided to Executive pursuant 
        to Article VII or Article VIII.


                                      IX

Sections 9.1 through 9.17 are redesignated as Sections 10.1 through 10.17,
respectively, and Sections 8.1 through 8.6 are redesignated as Sections 9.1
through 9.6, respectively.


                                       X

    Section 9.2(a) is amended by substituting "8.2(c)" for "9.2(c)" at the end
of such Section.


                                      XI

    Section 9.2(b) is amended by substituting "held on the Agreement Date" for
each reference to "now held" in the last sentence of such Section.


                                      XII

    Sections 9.4(a), 9.4(b) and 9.5(a) are each amended by substituting "9.1,
9.2 and 9.3" for each reference to "8.1, 8.2 and 8.3" included therein.


                                     XIII

    Sections 9.5(b), 9.5(c) and 10.17 are each amended by substituting "Article
IX" for all references to "Article VIII" therein.


                                      XIV

    Section 10.10 is amended by substituting "9.1, 9.6 and 10.9" for the
reference to "8.1, 8.6 and 9.9" included therein.

                                     -13-
<PAGE>
 
                                      XV

    Except as amended hereby, the Existing Employment Agreement, as previously
amended, remains in full force and effect.

 
                                      XVI

    This Third Amendment may be executed in any number of counterparts, each of
which shall be deemed to be an original, but all of which together shall be
deemed to be one and the same instrument.

    IN WITNESS WHEREOF, the parties have executed this Third Amendment as of the
first date written above.


                        UNICOM CORPORATION



                        By:________________________________________
                                     Edward A. Brennan,
                                Chairman of the Compensation
                                Committee of the Board of Directors


                        By:________________________________________
                                     Donald P. Jacobs
                                Lead Non-employee Director


                        COMMONWEALTH EDISON COMPANY



                        By:________________________________________
                                     Edward A. Brennan,
                                Chairman of the Compensation
                                Committee of the Board of Directors


                        By:________________________________________
                                     Donald P. Jacobs
                                Lead Non-employee Director



                        EXECUTIVE:



                        ______________________________________           
                                 John W. Rowe

                                     -14-