Exhibit 4.2
                             ILLINOIS POWER COMPANY


                                       TO

                         HARRIS TRUST AND SAVINGS BANK,

                                   as Trustee

                              ------------------


                             Supplemental Indenture

                            DATED AS OF JUNE 15, 1999

                                       TO

                  General Mortgage Indenture and Deed of Trust

                          DATED AS OF NOVEMBER 1, 1992

                                       75


Supplemental Indenture dated as of June 15, 1999 (the "Supplemental Indenture"),
made by and between ILLINOIS POWER COMPANY, a corporation organized and existing
under  the laws of the State of  Illinois  (the  "Company"),  party of the first
part,  and HARRIS TRUST AND SAVINGS BANK, a  corporation  organized and existing
under the laws of the State of Illinois  (the  "Trustee"),  as Trustee under the
General  Mortgage  Indenture  and Deed of Trust  dated as of  November  1, 1992,
hereinafter mentioned, party of the second part;

         WHEREAS,  the Company has heretofore executed and delivered its General
Mortgage  Indenture  and  Deed of  Trust  dated  as of  November  1,  1992  (the
"Indenture"),  to the  Trustee,  for the  security  of the Bonds of the  Company
issued and to be issued thereunder (the "Bonds"); and

         WHEREAS,  pursuant to the terms and  provisions of the Indenture  there
were created and  authorized  by  Supplemental  Indentures  thereto  bearing the
following  dates,  respectively,  the New  Mortgage  Bonds of the series  issued
thereunder and respectively identified opposite such dates:

Date of Supplemental   Identification
   Indenture              of Series                        Called
- --------------------   --------------                      ------
February 15, 1993   8% Series due 2023          Bonds of the 2023 Series
March 15, 1993      6 1/2% Series due 2000      Bonds of the 2000 Series
March 15, 1993      6 3/4% Series due 2005      Bonds of the 2005 Series
July 15, 1993       7 1/2% Series due 2025      Bonds of the 2025 Series
August 1, 1993      6 1/2% Series due 2003      Bonds of the First 2003 Series
October 15, 1993    5 5/8% Series due 2000      Bonds of the Second 2000 Series
November 1, 1993    Pollution Control Series M  Bonds of the Pollution Control
                                                Series M
November 1, 1993    Pollution Control Series N  Bonds of the Pollution Control
                                                Series N
November 1, 1993    Pollution Control Series O  Bonds of the Pollution Control
                                                Series O
April 1, 1997       Pollution Control Series P  Bonds of the Pollution Control
                                                Series P
April 1, 1997       Pollution Control Series Q  Bonds of the Pollution Control
                                                Series Q
April 1, 1997       Pollution Control Series R  Bonds of the Pollution Control
                                                Series R M
March 1, 1998       Pollution Control Series S  Bonds of the Pollution Control
                                                Series S
March 1, 1998       Pollution Control Series T  Bonds of the Pollution Control
                                                Series T
July 15, 1998       6 1/4% Series due 2002      Bonds of the 2002 Series
September 15, 1998  6% Series due 2003          Bonds of the Second 2003 Series


         WHEREAS,  the  Company  desires  to create a new  series of Bonds to be
issued under the Indenture,  to be known as New Mortgage Bonds, 7.50% Series due
2009 (the "New Mortgage Bonds of the 2009 Series"); and

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

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

         NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE WITNESSETH:

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         THAT  Illinois  Power  Company,  in  consideration  of the purchase and
ownership from time to time of the Bonds and the service by the Trustee, and its
successors, under the Indenture and of One Dollar to it duly paid by the Trustee
at or before the ensealing and delivery of these  presents,  the receipt whereof
is hereby acknowledged,  hereby covenants and agrees to and with the Trustee and
its  successors in the trust under the  Indenture,  for the benefit of those who
shall hold the Bonds as follows:


                                   ARTICLE I.

              DESCRIPTION OF NEW MORTGAGE BONDS OF THE 2009 SERIES.

         SECTION 1. The Company hereby creates a new series of Bonds to be known
as the "New Mortgage  Bonds of the 2009  Series." The New Mortgage  Bonds of the
2009 Series shall be executed,  authenticated  and delivered in accordance  with
the  provisions  of, and shall in all  respects be subject to, all of the terms,
conditions and covenants of the Indenture, as supplemented and modified.

         The  commencement  of the first interest period shall be June 29, 1999.
All New Mortgage  Bonds of the 2009 Series  shall  mature on June 15, 2009,  and
shall bear  interest  at the rate of SEVEN AND  ONE-HALF  PER CENT  (7.50%)  per
annum, payable semi-annually on June 15 and December 15 in each year, commencing
December  15,1999,  until the principal sum is paid in full. The person in whose
name any of the New  Mortgage  Bonds of the 2009  Series are  registered  at the
close of business on any record date (as  hereinafter  defined)  with respect to
any interest  payment date shall be entitled to receive the interest  payable on
such interest payment date notwithstanding the cancellation of such New Mortgage
Bonds of the 2009 Series upon any transfer or exchange  subsequent to the record
date and prior to such interest payment date; provided,  however, that if and to
the extent the Company  shall default in the payment of the interest due on such
interest  payment date,  such  defaulted  interest  shall be paid as provided in
Section 3.07 of the Indenture.

         The term  "record  date" as used in this  Section  with  respect to any
interest  payment  date shall mean the June 1 or December 1, as the case may be,
next  preceding the  semi-annual  interest  payment date,  or, if such June 1 or
December 1 shall be a legal  holiday or a day on which banking  institutions  in
the City of Chicago,  Illinois,  are  authorized by law to close,  then the next
preceding  day  which  shall  not be a  legal  holiday  or a day on  which  such
institutions are so authorized to close.

         SECTION 2. The New  Mortgage  Bonds of the 2009 Series  shall be issued
only as registered Bonds without coupons of the  denomination of $1,000,  or any
integral multiple of $1,000,  appropriately  numbered. The New Mortgage Bonds of
the 2009 Series may be exchanged,  upon surrender thereof,  at the agency of the
Company in the City of Chicago,  Illinois, for one or more New Mortgage Bonds of
the  2009  Series  of other  authorized  denominations,  for the same  aggregate
principal  amount,  subject  to  the  terms  and  conditions  set  forth  in the
Indenture.

         New Mortgage  Bonds of the 2009 Series may be exchanged or  transferred
without  expense to the registered  owner thereof except that any taxes or other
governmental  charges  required  to be paid with  respect  to such  transfer  or
exchange  shall be paid by the  registered  owner  requesting  such  transfer or
exchange as a condition precedent to the exercise of such privilege.

         SECTION 3. The New Mortgage Bonds of  the 2009 Series and the Trustee's
Certificate  of  Authentication  shall be  substantially in  the following forms
respectively:

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                             [FORM OF FACE OF BOND]

                             ILLINOIS POWER COMPANY
             (Incorporated under the laws of the State of Illinois)

                    NEW MORTGAGE BOND, 7.50% SERIES DUE 2009

No                                                                  $250,000,000

         ILLINOIS POWER COMPANY, a corporation  organized and existing under the
laws of the State of  Illinois  (the  "Company,"  which term shall  include  any
successor  corporation as defined in the Indenture hereinafter referred to), for
value received,  hereby promises to pay to Cede & Co. or registered assigns, the
principal sum of Two Hundred and Fifty Million  Dollars  ($250,000,000)  on June
15, 2009,  in any coin or currency of the United  States of America which at the
time of  payment  is legal  tender for  public  and  private  debts,  and to pay
interest  thereon  in  like  coin  or  currency  from  June  29,  1999,  payable
semi-annually on June 15 and December 15 in each year,  commencing  December 15,
1999,  at the rate of SEVEN AND ONE-HALF  PER CENT (7.50%) per annum,  until the
Company's  obligation  with  respect to the payment of such  principal  shall be
discharged as provided in the Indenture hereinafter  mentioned.  The interest so
payable on any June 15 or  December  15,  will,  subject  to certain  exceptions
provided in the Supplemental Indenture dated as of June 15, 1999, be paid to the
person  in whose  name  this New  Mortgage  Bond is  registered  at the close of
business on the immediately  preceding June 1 or December 1, as the case may be.
Both  principal  of, and interest on, this New Mortgage  Bond are payable at the
agency of the Company in the City of Chicago, Illinois.

         This New Mortgage  Bond shall not be entitled to any benefit  under the
Indenture or any indenture  supplemental  thereto, or become valid or obligatory
for any purpose,  until the form of certificate  endorsed hereon shall have been
signed by or on behalf of Harris Trust and Savings  Bank,  the Trustee under the
Indenture, or a successor trustee thereto under the Indenture (the "Trustee").

         The  provisions  of this New Mortgage Bond are continued on the reverse
hereof and such continued provisions shall for all purposes have the same effect
as though fully set forth at this place.

                      Illinois Commerce Commission No. 6120

         IN WITNESS WHEREOF, Illinois Power Company has caused this New Mortgage
Bond  to be  signed  (manually  or by  facsimile  signature)  in its  name by an
Authorized  Executive  Officer,  as defined in the Indenture,  and its corporate
seal (or a facsimile  thereof) to be hereto affixed and attested (manually or by
facsimile  signature)  by an  Authorized  Executive  Officer,  as defined in the
Indenture.

Dated:                                           ILLINOIS POWER COMPANY,



                                                 By:
                                                     ---------------------------
                                                    Authorized Executive Officer

(Corporate Seal)

ATTEST:


- -----------------------------------
   Authorized Executive Officer

                                       78


                [FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION]

         This New  Mortgage  Bond is one of the Bonds of the  series  designated
therein  referred to in the  within-mentioned  Indenture dated as of November 1,
1992 and the Supplemental Indenture dated as of June 15, 1999.


                                                 HARRIS TRUST AND SAVINGS BANK,
                                                     Trustee


                                                 By:
                                                    ----------------------------
                                                       Authorized Signatory


                            [FORM OF REVERSE OF BOND]

         This New Mortgage  Bond is one of a duly  authorized  issue of Bonds of
the Company (the "Bonds") in unlimited aggregate principal amount, of the series
hereinafter specified,  all issued and to be issued under and equally secured by
a General Mortgage  Indenture and Deed of Trust (the  "Indenture"),  dated as of
November 1, 1992,  executed by the Company to Harris Trust and Savings Bank (the
"Trustee"),  as Trustee,  to which  Indenture  and all  indentures  supplemental
thereto  reference is hereby made for a description of the properties  mortgaged
and pledged,  the nature and extent of the  security,  the rights of  registered
owners of the Bonds and of the  Trustee  in respect  thereof,  and the terms and
conditions  upon which the Bonds are, and are to be,  secured.  The Bonds may be
issued in series, for various principal sums, may mature at different times, may
bear  interest  at  different  rates and may  otherwise  vary as provided in the
Indenture.  This  New  Mortgage  Bond  of the  2009  Series  is one of a  series
designated as the "New Mortgage Bonds, 7.50% Series Due 2009" (the "New Mortgage
Bonds of the 2009  Series") of the Company,  unlimited  in  aggregate  principal
amount,  issued  under  and  secured  by  the  Indenture  and  described  in the
supplemental  indenture dated as of June 15, 1999 (the  "Supplemental  Indenture
dated as of June 15, 1999), between the Company and the Trustee, supplemental to
the Indenture.

         The New Mortgage  Bonds of the 2009 Series are subject to redemption on
the terms and subject to the conditions set forth in the Supplemental  Indenture
dated as of June 15, 1999.

         In case an Event of Default, as defined in the Indenture,  shall occur,
the principal of all the Bonds at any such time outstanding  under the Indenture
may be declared or may become due and payable,  upon the  conditions  and in the
manner and with the effect  provided in the  Indenture.  The Indenture  provides
that such declaration may be rescinded under certain circumstances.


                                   ARTICLE II.

                 ISSUE OF NEW MORTGAGE BONDS OF THE 2009 SERIES.

         SECTION  1. The  Company  hereby  exercises  the  right to  obtain  the
authentication  of $250,000,000  principal amount of Bonds pursuant to the terms
of Section 4.02 of the Indenture.  All such Bonds shall be New Mortgage Bonds of
the 2009 Series.

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         SECTION  2.  Such  New  Mortgage  Bonds  of  the  2009  Series  may  be
authenticated  and  delivered  prior  to the  filing  for  recordation  of  this
Supplemental Indenture.


                                  ARTICLE III.

                                   REDEMPTION.

         The  Company at its option may,  at any time,  redeem the New  Mortgage
Bonds of the  2009  Series,  in whole or in part (if in part,  by lot or by such
other method as the Trustee shall deem fair or  appropriate)  prior to maturity,
on any date, upon payment of a redemption price equal to the greater of (i) 100%
of the  principal  amount  of the New  Mortgage  Bonds of the 2009  Series to be
redeemed  plus  accrued  and  unpaid  interest  thereon,  if any,  from the last
interest  payment date to the date of redemption,  or (ii) the Make Whole Amount
plus accrued and unpaid interest, if any, from the last interest payment date to
the redemption date.

         "Make Whole Amount"  means,  with respect to a New Mortgage Bond of the
2009  Series  at any  time,  the  sum of the  present  values  of the  Remaining
Scheduled Payments (as defined below) discounted, on a semiannual basis assuming
a 360-day  year  consisting  of twelve  30-day  months),  at a rate equal to the
Treasury  Rate (as defined  below) plus 20 basis  points.  The Make Whole Amount
shall  be  computed  as of the  third  Business  Day  prior  to  the  applicable
redemption date, and certified, by an Investment Banker (as defined below).

         "Investment Banker" means an independent investment banking institution
of good standing selected by the Company.

         "Remaining Scheduled Payments" means the remaining scheduled payment of
the principal  and interest that would be due if such New Mortgage  Bonds of the
2009  Series  were  not  redeemed.  However,  if the  redemption  date  is not a
scheduled  interest  payment date, the amount of the next  succeeding  scheduled
interest payment on such New Mortgage Bond of the 2009 Series will be reduced by
the amount of interest  accrued on such New Mortgage Bonds of the 2009 Series to
such redemption date.

         "Treasury Rate" means an annual rate equal to the semiannual equivalent
yield to maturity of the Comparable Treasury Issue (as defined below),  assuming
a price for the  Comparable  Treasury  Issue  (expressed  as a percentage of its
principal amount) equal to the Comparable  Treasury Price (as defined below) for
the  redemption  date.  The  semiannual  equivalent  yield to  maturity  will be
computed as of the third Business Day immediately preceding the redemption date.

         "Comparable  Treasury Issue" means the United States Treasury  security
selected by Salomon Smith Barney Inc. or J.P.  Morgan  Securities  Inc. or their
affiliates  as having a maturity  comparable  to the  remaining  term of the New
Mortgage  Bonds of the  2009  Series  that  would  be  utilized,  at the time of
selection and in accordance with customary  financial  practice,  in pricing new
issues of corporate debt securities of comparable maturity to the remaining term
of the New Mortgage Bonds of the 2009 Series.

         "Comparable  Treasury  Price"  means  the  average  of three  Reference
Treasury  Dealer  Quotations (as defined below)  obtained by the Trustee for the
redemption date.

         "Reference  Treasury  Dealers" means Salomon Smith Barney Inc. and J.P.
Morgan Securities Inc. (so long as they continue to be primary U.S. Government
securities dealers) and

                                       80


any one other primary U.S.  Government  securities dealer chosen by the Company.
If either Salomon Smith Barney Inc. or J.P. Morgan  Securities Inc. ceases to be
a primary U.S.  Government  securities  dealer,  the Company will appoint in its
place another  nationally  recognized  investment banking firm that is a primary
U.S. Government securities dealer.

         "Reference  Treasury Dealer Quotation" means the average, as determined
by the Trustee,  of the bid and asked prices for the  Comparable  Treasury Issue
(expressed  in each case as a  percentage  of its  principal  amount)  quoted in
writing to the Trustee by a  Reference  Treasury  Dealer at 3:30 p.m.,  New York
City time, on the third Business Day preceding the redemption date.


                                   ARTICLE IV.

                             AMENDMENT OF INDENTURE.

         Section  7.07(a)(iii)(A)  is hereby  amended by  inserting in the fifth
line thereof the words "and all Retired Bonds"  immediately  following the words
"Bonds then Outstanding."


                                   ARTICLE V.

                                  THE TRUSTEE.

         The Trustee hereby accepts the trusts hereby declared and provided, and
agrees to perform the same upon the terms and  conditions  in the  Indenture set
forth and upon the following terms and conditions:

                  The Trustee shall not be responsible in any manner  whatsoever
         for or in respect of the validity or sufficiency  of this  Supplemental
         Indenture  or the due  execution  hereof  by the  Company  or for or in
         respect of the recitals  contained  herein,  all of which  recitals are
         made by the  Company  solely.  In  general,  each  and  every  term and
         condition  contained in Article Eleven of the Indenture  shall apply to
         this  Supplemental  Indenture  with the same force and effect as if the
         same were herein set forth in full, with such omissions, variations and
         modifications thereof as may be appropriate to make the same conform to
         this Supplemental Indenture.

                                   ARTICLE VI.

                            MISCELLANEOUS PROVISIONS.

         This  Supplemental  Indenture  may be  simultaneously  executed  in any
number of counterparts,  each of which when so executed shall be deemed to be an
original;  but such counterparts shall together  constitute but one and the same
instrument.

                                       81


         IN WITNESS WHEREOF, Illinois Power Company has caused this Indenture to
be executed on its behalf by an Authorized  Executive  Officer as defined in the
Indenture,  and its corporate  seal to be hereto  affixed and said seal and this
Indenture to be attested by an  Authorized  Executive  Officer as defined in the
Indenture; and said Harris Trust and Savings Bank, in evidence of its acceptance
of the trust  hereby  created,  has caused this  Indenture to be executed on its
behalf by its President or one of its Vice  Presidents and its corporate seal to
be  hereto  affixed  and said  seal and this  Indenture  to be  attested  by its
Secretary or one of its Assistant Secretaries,  all as of the date first written
above.

                                                 ILLINOIS POWER COMPANY


                                                 By
                                                   -----------------------------
(CORPORATE SEAL)                                       Robert A. Schultz
                                                       Vice President - Finance


ATTEST:


- ------------------------------
Leah Manning Stetzner
Corporate Secretary








                                                  HARRIS TRUST AND SAVINGS BANK,
                                                   Trustee


                                                  By
                                                    ----------------------------
                                                       J. Bartolini
                                                       Vice President


(CORPORATE SEAL)

ATTEST:


- ----------------------------
C. Potter
Assistant Secretary

                                       82


STATE OF ILLINOIS    )
                     )SS.:
COUNTY OF MACON      )

         BE IT REMEMBERED,  that on this ___ day of _____,  1999, before me, the
undersigned,  a Notary  Public  within and for the  County and State  aforesaid,
personally  came Robert A.  Schultz,  Vice  President - Finance and Leah Manning
Stetzner,  Corporate  Secretary,  of Illinois Power Company,  a corporation duly
organized,  incorporated  and existing  under the laws of the State of Illinois,
who are personally known to me to be such officers, and who are personally known
to me to be the same persons who executed as such officers the within instrument
of writing,  and such persons  duly  acknowledged  that they signed,  sealed and
delivered the said  instrument as their free and voluntary act as such officers,
and as the free and voluntary  act of said  Illinois  Power Company for the uses
and purposes therein set forth.

         IN WITNESS WHEREOF,  I have hereunto  subscribed my name and affixed my
official seal on the day and year last above written.




                                           Notary Public, Macon County, Illinois


My Commission Expires _______________.

(NOTARIAL SEAL)



STATE OF ILLINOIS    )
                     )SS.:
COUNTY OF COOK       )

         BE IT REMEMBERED, that on this ____ day of ______, 1999, before me, the
undersigned,  a Notary  Public  within and for the  County and State  aforesaid,
personally came J. Bartolini, Vice President and C. Potter, Assistant Secretary,
of Harris Trust and Savings Bank, a corporation duly organized, incorporated and
existing under the laws of the State of Illinois, who are personally known to me
to be such officers,  and who are personally  known to me to be the same persons
who executed as such officers the within instrument of writing, and such persons
duly acknowledged that they signed,  sealed and delivered the said instrument as
their free and voluntary act as such officers, and as the free and voluntary act
of said Harris  Trust and  Savings  Bank for the uses and  purposes  therein set
forth.

         IN WITNESS WHEREOF,  I have hereunto  subscribed my name and affixed my
official seal on the day and year last above written.



                                            Notary Public, Cook County, Illinois

My Commission Expires _____________.

(NOTARIAL SEAL)

                                       83


Return To:                                    This Instrument Was Prepared By:

   ILLINOIS POWER COMPANY                     SCHIFF HARDIN & WAITE
   Real Estate Dept. F-14                     6600 Sears Tower
   500 S. 27th Street                         233 South Wacker Drive
   Decatur, IL 62525                          Chicago, IL 60606

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