ILLINOIS POWER CO
SC 13E4/A, 1998-04-02
ELECTRIC & OTHER SERVICES COMBINED
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<PAGE>   1
 
- --------------------------------------------------------------------------------
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                               ------------------
 
                         Issuer Tender Offer Statement
     (Pursuant to Section 13(e)(1) of the Securities Exchange Act of 1934)
 
   
                               (AMENDMENT NO. 1)
    
 
                             ILLINOIS POWER COMPANY
                                (Name of Issuer)
 
                              ILLINOVA CORPORATION
                       (Name of Person Filing Statement)
 
                               ------------------
 
<TABLE>
<CAPTION>
                       TITLE                                           CUSIP NUMBER
                       -----                                           ------------
<S>                                                 <C>
    CUMULATIVE PREFERRED STOCK ($50 PAR VALUE)
                   4.08% SERIES                                         452092 20 8
                   4.20% SERIES                                         452092 30 7
                   4.26% SERIES                                         452092 40 6
                   4.42% SERIES                                         452092 50 5
                   4.70% SERIES                                         452092 60 4
                   7.75% SERIES                                         452092 79 4
                            (Title and CUSIP Number of Class of Securities)
</TABLE>
 
                          LEAH MANNING STETZNER, ESQ.
                    GENERAL COUNSEL AND CORPORATE SECRETARY
                              ILLINOVA CORPORATION
                               500 S. 27TH STREET
                               DECATUR, IL 62525
                                 (217) 424-6600
 
                                with a copy to:
 
                             ROBERT J. REGAN, ESQ.
                             SCHIFF HARDIN & WAITE
                                7200 SEARS TOWER
                            CHICAGO, ILLINOIS 60616
                                 (312) 258-5606
 
  (Name, Address and Telephone Number of Person Authorized to Receive Notices
          and Communications on Behalf of the Person Filing Statement)
 
                               ------------------
 
                              ON OR ABOUT   , 1998
     (Date Tender Offer First Published, Sent or Given to Security Holders)
 
                               ------------------
 
                           CALCULATION OF FILING FEE
- --------------------------------------------------------------------------------
TRANSACTION VALUATION*  $52,607,718.80          AMOUNT OF FILING FEE  $10,521.54
- --------------------------------------------------------------------------------
* Solely for purposes of calculating the filing fee and computed pursuant to
  Section 13(e)(3) of the Securities Exchange Act of 1934, as amended, and Rule
  0-11(b)(1) thereunder, the transaction value equals the total amount of funds,
  excluding fees and other expenses, required to purchase all outstanding shares
  of each class of securities listed above pursuant to the Offer described in
  the Offer to Purchase and Proxy Statement filed as Exhibit 99(a)(1) hereto.
  The purchase price per share for the outstanding shares will be based on a
  formula utilizing the 30-year Treasury Bond rate (except that the 5-year
  Treasury Bond rate will be used with respect to the 7.75% Series) at the
  commencement of the Offer. The above transaction value is based on the formula
  utilizing the 30-year and 5-year Treasury Bond rates as of February 25, 1998.
  Any adjustments to the filing fee will be made upon filing an Amendment to
  this Schedule 13E-4 upon commencement of the Offer.
 
  [X] Check box if any part of the fee is offset as provided by Rule 0-11(a)(2)
      and identify the filing with which the offsetting fee was previously paid.
      Identify the previous filing by registration statement number, or the form
      or schedule and the date of its filing.
 
   
<TABLE>
<S>                                                 <C>
Amount Previously Paid: $10,521.54                  Filing Party: Illinova Corporation
Form or Registration No.: Schedule 13E-4            Date Filed: March 3, 1998
</TABLE>
    
 
- --------------------------------------------------------------------------------
<PAGE>   2
 
ITEM 1. SECURITY AND ISSUER.
 
     (a) Incorporated herein by reference to the information appearing on the
front cover of the Offer to Purchase and Proxy Statement, dated                ,
1998, filed as Exhibit 99(a)(1) to this Issuer Tender Offer Statement on
Schedule 13E-4 (the "Offer to Purchase and Proxy Statement").
 
     (b) Incorporated herein by reference to the information appearing on the
front cover of the Offer to Purchase and Proxy Statement, and to the information
appearing under the captions "Terms of the Offer -- Number of Shares; Purchase
Prices; Expiration Date; Dividends" and "Transactions and Agreements Concerning
the Shares" in the Offer to Purchase and Proxy Statement.
 
     (c) Incorporated herein by reference to the information appearing under the
caption "Price Range of Shares; Dividends" in the Offer to Purchase and Proxy
Statement.
 
     (d) Illinova Corporation, an Illinois corporation ("ILN"), is the person
filing this statement and is the parent holding company of the Issuer, Illinois
Power Company ("IPC"). ILN's principal office is at 500 S. 27th Street, Decatur,
IL 62525.
 
ITEM 2. SOURCE AND AMOUNT OF FUNDS OR OTHER CONSIDERATION.
 
     (a) Incorporated herein by reference to the information appearing under the
caption "Source and Amount of Funds" in the Offer to Purchase and Proxy
Statement.
 
     (b) Incorporated herein by reference to the information appearing under the
caption "Source and Amount of Funds" in the Offer to Purchase and Proxy
Statement.
 
ITEM 3. PURPOSE OF THE TENDER OFFER AND PLANS OR PROPOSALS OF THE ISSUER OR
AFFILIATE.
 
     Incorporated herein by reference to the information appearing under the
captions "Special Factors -- Purpose of the Offer; Certain Effects of the Offer;
Plans of ILN and IPC After the Offer" and "Proposed Amendment and Proxy
Solicitation -- Reasons for the Proposed Amendment" in the Offer to Purchase and
Proxy Statement.
 
ITEM 4. INTEREST IN SECURITIES OF THE ISSUER.
 
     Incorporated herein by reference to the information appearing under the
caption "Transactions and Agreements Concerning the Shares" in the Offer to
Purchase and Proxy Statement.
 
ITEM 5. CONTRACTS, ARRANGEMENTS, UNDERSTANDINGS OR RELATIONSHIPS WITH RESPECT TO
        THE ISSUER'S SECURITIES.
 
     Incorporated herein by reference to the information appearing under the
caption "Transactions and Agreements Concerning the Shares" in the Offer to
Purchase and Proxy Statement.
 
ITEM 6. PERSONS RETAINED, EMPLOYED OR TO BE COMPENSATED.
 
     Incorporated herein by reference to the information appearing under the
caption "Fees and Expenses Associated with the Offer" in the Offer to Purchase
and Proxy Statement.
 
ITEM 7. FINANCIAL INFORMATION.
 
   
     (a) Incorporated herein by reference to IPC's financial statements included
in the Annual Report on Form 10-K for the year ended December 31, 1997,
referenced herein as Exhibit 99(g)(1) and to the information appearing under the
caption "Summary of Consolidated Financial Information" in the Offer to Purchase
and Proxy Statement.
    
 
     (b) Not applicable.
 
                                        2
<PAGE>   3
 
ITEM 8. ADDITIONAL INFORMATION.
 
     (a) Not applicable.
 
     (b) Incorporated herein by reference to the information appearing under the
caption "Special Factors -- Certain Legal Matters; Regulatory Approvals;
Dissenters' Rights" and "Terms of the Offer -- Certain Conditions of the Offer"
in the Offer to Purchase and Proxy Statement.
 
     (c) Not applicable.
 
     (d) Not applicable.
 
     (e) See Exhibits 99(a)(1) and 99(a)(2).
 
ITEM 9. MATERIAL TO BE FILED AS EXHIBITS.
 
   
<TABLE>
<CAPTION>
    EXHIBIT NO.                            DESCRIPTION
    -----------                            -----------
    <S>            <C>
    99(a)(1)       Offer to Purchase and Proxy Statement, dated             ,
                   1998.
    99(a)(2)       Form of Letter of Transmittal and Proxy.
    99(a)(3)       Form of Separate Proxy.
    99(a)(4)       Form of Notice of Guaranteed Delivery.
    99(a)(5)       Notice of Special Meeting of Shareholders.
    99(a)(6)       Form of Letter to Brokers, Dealers, Commercial Banks, Trust
                   Companies and Other Nominees.
    99(a)(7)       Form of Letter to Clients of Brokers, Dealers, Commercial
                   Banks, Trust Companies and Other Nominees.
    99(a)(8)       Letter to Shareholders, dated             , 1998.
    99(a)(9)       Summary Advertisement, dated             , 1998.*
    99(a)(10)      Press Release, dated             , 1998.*
    99(b)          Credit Agreement, dated as of June 12, 1996, as amended on
                   June 28, 1996, among ILN, CIBC Inc., as Administrative
                   Agent, The First National Bank of Chicago, Bank of America
                   Illinois, The First National Bank of Boston, Bank of
                   Montreal, The Fuji Bank, Limited, ABN AMRO Bank N.V. Chicago
                   Branch, The Bank of New York and Deutsche Bank A.G. New York
                   Branch and/or Cayman Islands Branch.**
    99(c)          Not applicable.
    99(d)          Not applicable.
    99(e)          Not applicable.
    99(f)          Not applicable.
    99(g)(1)       Annual Report on Form 10-K of IPC for the year ended
                   December 31, 1997, incorporated by reference to SEC File No.
                   1-3004.
</TABLE>
    
 
- -------------------------
 * To be filed by amendment.
   
** Previously filed.
    
 
                                        3
<PAGE>   4
 
                                   SIGNATURE
 
     After due inquiry and to the best of my knowledge and belief, I certify
that the information set forth in this statement is true, complete and correct.
 
Dated: April 1, 1998                      ILLINOVA CORPORATION
 
                                          By: /s/ Leah Manning Stetzner
 
                                            ------------------------------------
                                            Leah Manning Stetzner,
                                            General Counsel and Corporate
                                              Secretary
 
                                        4

<PAGE>   1
 
OFFER TO PURCHASE AND PROXY STATEMENT
 
                              ILLINOVA CORPORATION
                           OFFER TO PURCHASE FOR CASH
           ANY AND ALL OUTSTANDING SHARES OF THE FOLLOWING SERIES OF
                         CUMULATIVE PREFERRED STOCK OF
 
                             ILLINOIS POWER COMPANY
283,290 SHARES, CUMULATIVE PREFERRED STOCK, 4.08% SERIES AT A PURCHASE PRICE OF
                     $  PER SHARE, CUSIP NUMBER 452092 20 8
167,720 SHARES, CUMULATIVE PREFERRED STOCK, 4.20% SERIES AT A PURCHASE PRICE OF
                     $  PER SHARE, CUSIP NUMBER 452092 30 7
136,000 SHARES, CUMULATIVE PREFERRED STOCK, 4.26% SERIES AT A PURCHASE PRICE OF
                     $  PER SHARE, CUSIP NUMBER 452092 40 6
134,400 SHARES, CUMULATIVE PREFERRED STOCK, 4.42% SERIES AT A PURCHASE PRICE OF
                     $  PER SHARE, CUSIP NUMBER 452092 50 5
176,000 SHARES, CUMULATIVE PREFERRED STOCK, 4.70% SERIES AT A PURCHASE PRICE OF
                     $  PER SHARE, CUSIP NUMBER 452092 60 4
241,700 SHARES, CUMULATIVE PREFERRED STOCK, 7.75% SERIES AT A PURCHASE PRICE OF
                     $  PER SHARE, CUSIP NUMBER 452092 79 4
                     -------------------------------------
 
                             ILLINOIS POWER COMPANY
                                PROXY STATEMENT
        WITH RESPECT TO ITS COMMON STOCK AND CUMULATIVE PREFERRED STOCK
                     -------------------------------------
 
     THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 5:00 P.M., NEW YORK CITY
TIME, ON [EXPIRATION DATE], UNLESS THE OFFER IS EXTENDED (THE "EXPIRATION
DATE").
   
    Illinova Corporation, an Illinois corporation ("ILN"), invites the holders
of each series of cumulative preferred stock (par value $50 per share) listed
above (each a "Series of Preferred," and the holder thereof a "Preferred
Shareholder") of Illinois Power Company, an Illinois corporation and direct
utility subsidiary of ILN ("IPC"), to tender any and all of their shares of a
Series of Preferred ("Shares") for purchase at the purchase price per Share
listed above (the "Purchase Price") plus accrued and unpaid dividends for the
Shares tendered, net to the seller in cash, upon the terms and subject to the
conditions set forth in this Offer to Purchase and Proxy Statement and in the
accompanying Letter of Transmittal and Proxy (which together constitute the
"Offer"). ILN will purchase any and all Shares validly tendered and not
withdrawn, upon the terms and subject to the conditions of the Offer. See "Terms
of the Offer -- Certain Conditions of the Offer" and "Terms of the Offer --
Extension of Tender Period; Termination; Amendments."
    
    THE OFFER FOR A SERIES OF PREFERRED IS NOT CONDITIONED UPON ANY MINIMUM
NUMBER OF SHARES OF SUCH SERIES OF PREFERRED BEING TENDERED AND IS INDEPENDENT
OF THE OFFER FOR ANY OTHER SERIES OF PREFERRED. THE OFFER, HOWEVER, IS
CONDITIONED UPON, AMONG OTHER THINGS, THE PROPOSED AMENDMENT, AS DESCRIBED
BELOW, BEING ADOPTED AT THE SPECIAL MEETING (AS DEFINED BELOW). SEE "TERMS OF
THE OFFER -- CERTAIN CONDITIONS OF THE OFFER."
   
    Concurrently with the Offer, the Board of Directors of IPC is soliciting
proxies for use at a special meeting of shareholders of IPC to be held at IPC's
principal office, 500 South 27th Street, Decatur, Illinois 62525, on [EXPIRATION
DATE] at 4:30 p.m., New York City time, or any adjournment or postponement of
such meeting (the "Special Meeting"). The purpose of the Special Meeting is to
consider an amendment (the "Proposed Amendment") to IPC's Amended and Restated
Articles of Incorporation (the "Articles") which would remove a provision of the
Articles that limits IPC's ability to issue or assume unsecured indebtedness
(the "Debt Limitation Provision"). WHILE PREFERRED SHAREHOLDERS WHO WISH TO
TENDER THEIR SHARES PURSUANT TO THE OFFER ARE NOT REQUIRED TO VOTE IN FAVOR OF
THE PROPOSED AMENDMENT, THE OFFER IS CONDITIONED UPON THE PROPOSED AMENDMENT
BEING ADOPTED AT THE SPECIAL MEETING. IN ADDITION, PREFERRED SHAREHOLDERS HAVE
THE RIGHT TO VOTE FOR THE PROPOSED AMENDMENT REGARDLESS OF WHETHER THEY TENDER
THEIR SHARES. IF THE PROPOSED AMENDMENT IS ADOPTED AT THE SPECIAL MEETING, IPC
WILL MAKE A SPECIAL CASH PAYMENT (AS DEFINED HEREIN) IN THE AMOUNT OF $    PER
SHARE TO EACH PREFERRED SHAREHOLDER OF RECORD AS OF [RECORD DATE] WHO DID NOT
TENDER HIS OR HER SHARES IN THE OFFER. THOSE PREFERRED SHAREHOLDERS WHO VALIDLY
TENDER THEIR SHARES WILL BE ENTITLED ONLY TO THE APPLICABLE PURCHASE PRICE PER
SHARE LISTED ABOVE PLUS ACCRUED AND UNPAID DIVIDENDS FOR THE SHARES TENDERED.
    
                     -------------------------------------
 
   
    ILN will pay to a Soliciting Dealer (as defined herein) a solicitation fee
for Shares tendered, accepted for payment and paid for pursuant to the Offer,
subject to certain conditions. If the Proposed Amendment is adopted at the
Special Meeting, IPC will pay to a Soliciting Dealer a separate fee for Shares
not tendered pursuant to the Offer but which are voted in favor of the Proposed
Amendment. See "Fees and Expenses Associated with the Offer."
    
                     -------------------------------------
 
    THIS TRANSACTION HAS NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
EXCHANGE COMMISSION (THE "SEC") NOR HAS THE SEC PASSED UPON THE FAIRNESS OR
MERITS OF THIS TRANSACTION NOR UPON THE ACCURACY OR ADEQUACY OF THE INFORMATION
CONTAINED IN THIS DOCUMENT. ANY REPRESENTATION TO THE CONTRARY IS UNLAWFUL.
                     -------------------------------------
 
    NEITHER ILN, IPC, THEIR RESPECTIVE BOARDS OF DIRECTORS, NOR ANY OF THEIR
RESPECTIVE OFFICERS MAKES ANY RECOMMENDATION TO ANY PREFERRED SHAREHOLDER AS TO
WHETHER TO TENDER ANY OR ALL SHARES. EACH PREFERRED SHAREHOLDER MUST MAKE HIS OR
HER OWN DECISION AS TO WHETHER TO TENDER SHARES AND, IF SO, HOW MANY SHARES TO
TENDER.
                     -------------------------------------
 
     IPC'S BOARD OF DIRECTORS RECOMMENDS VOTING FOR THE PROPOSED AMENDMENT.
                     -------------------------------------
   
    This Offer to Purchase and Proxy Statement is first being mailed on or about
[STATEMENT DATE] to Preferred Shareholders of record on [RECORD DATE].
    
   
    IPC has declared the regular quarterly dividend on each Series of Preferred
to be paid on May 1, 1998 (the "May 1998 Dividend") to holders of record as of
the close of business on April 9, 1998. A tender and purchase of Shares pursuant
to the Offer will not deprive a Preferred Shareholder of his or her right to
receive the May 1998 Dividend on his or her Shares held of record as of the
close of business on April 9, 1998, regardless of whether such Preferred
Shareholder tenders his or her Shares in the Offer prior to that date. Tendering
Preferred Shareholders will be entitled to any dividends accrued and unpaid
prior to, but not including, the Payment Date (as defined herein) in respect of
any later dividend periods (or any portion thereof).
    
    Each Series of Preferred is traded on the New York Stock Exchange, Inc. (the
"NYSE"), except the 7.75% Series which trades in the over-the-counter market
(the "OTC"). On [LAST SALE DATE REFERENCE], the last reported sales prices on
the NYSE were $    for the 4.08% Series of Preferred (on         ); $    for the
4.20% Series of Preferred (on       ); $    for the 4.26% Series of Preferred
(on         ); $
for the 4.42% Series of Preferred (on         ); and $    for the 4.70% Series
of Preferred (on         ). On [LAST SALE DATE REFERENCE], the last reported
sales price as reported by the National Quotation Bureau, Inc. was $    for the
7.75% Series of Preferred (on         ). Preferred Shareholders are urged to
obtain a current market quotation, if available, for their Shares.
                     -------------------------------------
                      THE DEALER MANAGER FOR THE OFFER IS:
 
                          DONALDSON, LUFKIN & JENRETTE
                                SECURITIES CORPORATION
  The date of this Offer to Purchase and Proxy Statement is [STATEMENT DATE].
<PAGE>   2
 
     NO PERSON HAS BEEN AUTHORIZED TO MAKE ANY RECOMMENDATION ON BEHALF OF ILN
OR IPC AS TO WHETHER PREFERRED SHAREHOLDERS SHOULD TENDER OR REFRAIN FROM
TENDERING SHARES OF ANY SERIES OF PREFERRED PURSUANT TO THE OFFER. NO PERSON HAS
BEEN AUTHORIZED TO GIVE ANY INFORMATION OR TO MAKE ANY REPRESENTATIONS IN
CONNECTION WITH THE OFFER OTHER THAN THOSE CONTAINED HEREIN, IN THE RELATED
LETTER OF TRANSMITTAL AND PROXY OR IN THE SEPARATE PROXY. IF GIVEN OR MADE, SUCH
RECOMMENDATION AND SUCH INFORMATION AND REPRESENTATIONS MUST NOT BE RELIED UPON
AS HAVING BEEN AUTHORIZED BY ILN OR IPC.
 
                                   IMPORTANT
 
     Any Preferred Shareholder desiring to accept the Offer and tender all or
any portion of his or her Shares should either (i) request his or her broker,
dealer, commercial bank, trust company or nominee to effect the transaction for
him or her, (ii) complete and sign the applicable Letter of Transmittal and
Proxy enclosed herewith (the "Letter of Transmittal and Proxy") in accordance
with the instructions in such Letter of Transmittal and Proxy, mail or deliver
the same and any other required documents to First Chicago Trust Company of New
York (the "Depositary"), and deliver the certificates for such Shares, together
with the Letter of Transmittal and Proxy, to the Depositary, or (iii) tender
such Shares pursuant to the procedure for book-entry transfer set forth below
under "Terms of the Offer -- Procedure for Tendering Shares," in each case on or
prior to the Expiration Date (as defined below). A Preferred Shareholder whose
Shares are registered in the name of a broker, dealer, commercial bank, trust
company or nominee must contact such broker, dealer, commercial bank, trust
company or nominee if he or she desires to tender such Shares. Any Preferred
Shareholder who desires to tender Shares and whose certificates for such Shares
are not immediately available, or who cannot comply in a timely manner with the
procedure for book-entry transfer, should tender such Shares by following the
procedures for guaranteed delivery set forth below under "Terms of the Offer --
Procedure for Tendering Shares."
 
   
     EACH SERIES OF PREFERRED HAS ITS OWN LETTER OF TRANSMITTAL AND PROXY AND
ITS OWN SEPARATE PROXY (THE "SEPARATE PROXY"). ONLY THE APPLICABLE LETTER OF
TRANSMITTAL AND PROXY FOR SUCH SERIES OF PREFERRED OR A NOTICE OF GUARANTEED
DELIVERY MAY BE USED TO TENDER SHARES OF SUCH SERIES OF PREFERRED. A PROXY
(WHETHER INCLUDED AS PART OF THE LETTER OF TRANSMITTAL AND PROXY OR AS A
SEPARATE PROXY ENCLOSED HEREWITH) MAY BE USED TO VOTE IN FAVOR OF THE PROPOSED
AMENDMENT EVEN IF NO SHARES ARE BEING TENDERED.
    
                           -------------------------
 
   
     Questions or requests for assistance may be directed to MacKenzie Partners,
Inc. ("MacKenzie" or the "Information Agent") or to Donaldson, Lufkin & Jenrette
Securities Corporation ("DLJ" or the "Dealer Manager") at their respective
telephone numbers and addresses set forth on the back cover of this Offer to
Purchase and Proxy Statement. Requests for additional copies of this Offer to
Purchase and Proxy Statement, the Letter of Transmittal and Proxy, the Separate
Proxy or other related tender offer or proxy materials may be directed to the
Information Agent, and such copies will be furnished promptly at ILN's or IPC's
expense, as applicable. Preferred Shareholders may also contact their local
broker, dealer, commercial bank or trust company for assistance concerning the
Offer.
    
 
                                        2
<PAGE>   3
 
                               TABLE OF CONTENTS
 
   
<TABLE>
<S>                                                             <C>
SUMMARY.....................................................      4
SPECIAL FACTORS.............................................      8
  Purpose of the Offer; Certain Effects of the Offer........      8
  Liquidity of Trading Market...............................     10
  Potential Effects of the Proposed Amendment on
     Non-Tendering Preferred Shareholders...................     10
  Plans of ILN and IPC After the Offer......................     10
  Certain Legal Matters; Regulatory Approvals...............     11
  Dissenters' Rights........................................     12
TERMS OF THE OFFER..........................................     12
  Number of Shares; Purchase Prices; Expiration Date;
     Dividends..............................................     12
  Procedure for Tendering Shares............................     13
  Withdrawal Rights.........................................     15
  Acceptance of Shares for Payment and Payment of Purchase
     Price..................................................     16
  Certain Conditions of the Offer...........................     16
  Extension of Tender Period; Termination; Amendments.......     18
PROPOSED AMENDMENT AND PROXY SOLICITATION...................     19
  Introduction..............................................     19
  Voting Securities, Rights and Procedures..................     19
  Dissenters' Rights........................................     19
  Proxies...................................................     20
  Special Cash Payments.....................................     21
  Security Ownership of Certain Beneficial Owners and
     Management.............................................     21
  Business to Come Before the Special Meeting...............     23
  Explanation of the Proposed Amendment.....................     23
  Reasons for the Proposed Amendment........................     23
  Relationship with Independent Accountants.................     25
PRICE RANGE OF SHARES; DIVIDENDS............................     25
CERTAIN FEDERAL INCOME TAX CONSEQUENCES.....................     27
SOURCE AND AMOUNT OF FUNDS..................................     29
TRANSACTIONS AND AGREEMENTS CONCERNING THE SHARES...........     29
FEES AND EXPENSES ASSOCIATED WITH THE OFFER.................     29
CERTAIN INFORMATION REGARDING ILN AND IPC; INCORPORATION BY
  REFERENCE.................................................     31
SUMMARY OF CONSOLIDATED FINANCIAL INFORMATION; INCORPORATION
  BY REFERENCE..............................................     32
SHAREHOLDER PROPOSALS.......................................     34
MISCELLANEOUS...............................................     34
ATTACHMENT A................................................    A-1
</TABLE>
    
 
                                        3
<PAGE>   4
 
                                    SUMMARY
 
   
     The following summary is provided solely for the convenience of the
Preferred Shareholders. This summary is not intended to be complete and is
qualified in its entirety by reference to the full text and more specific
details contained in this Offer to Purchase and Proxy Statement, the Letter of
Transmittal and Proxy and the Separate Proxy and any amendments hereto or
thereto. Preferred Shareholders are urged to read this Offer to Purchase and
Proxy Statement, the Letter of Transmittal and Proxy and the Separate Proxy in
their entirety. Each of the capitalized terms used in this summary and not
defined herein has the meaning set forth elsewhere in this Offer to Purchase and
Proxy Statement.
    
 
The Companies.................   Illinova Corporation ("ILN"), 500 South 27th
                                 Street, Decatur, Illinois 62525, is an exempt
                                 holding company under the Public Utility
                                 Holding Company Act of 1935, as amended (the
                                 "Holding Company Act"). ILN has three principal
                                 operating subsidiaries: Illinois Power Company
                                 ("IPC"), a combination electric and gas
                                 utility; Illinova Generating Company, an
                                 independent power supply company; and Illinova
                                 Energy Partners, Inc., an energy broker and
                                 services company. IPC is engaged in the
                                 generation, transmission, distribution and sale
                                 of electric energy and the distribution,
                                 transportation and sale of natural gas in the
                                 State of Illinois. Its service area is a widely
                                 diversified industrial and agricultural area
                                 comprising approximately 15,000 square miles in
                                 northern, central and southern Illinois.
                                 Electric service is provided at retail to 310
                                 incorporated municipalities, adjacent suburban
                                 and rural areas and numerous unincorporated
                                 municipalities. Gas service is provided to 257
                                 incorporated municipalities, adjacent suburban
                                 areas and numerous unincorporated
                                 municipalities.
 
The Shares....................   4.08% Series, Cumulative Preferred Stock (par
                                 value $50 per share)
                                 4.20% Series, Cumulative Preferred Stock (par
                                 value $50 per share)
                                 4.26% Series, Cumulative Preferred Stock (par
                                 value $50 per share)
                                 4.42% Series, Cumulative Preferred Stock (par
                                 value $50 per share)
                                 4.70% Series, Cumulative Preferred Stock (par
                                 value $50 per share)
                                 7.75% Series, Cumulative Preferred Stock (par
                                 value $50 per share)
 
The Offer and Purchase
Price.........................   Offer to purchase any or all Shares of each
                                 Series of Preferred at the prices set forth
                                 below, which prices shall be the "Purchase
                                 Price" with respect to the applicable Series of
                                 Preferred.
 
                                 $  per Share of the 4.08% Series
                                 $  per Share of the 4.20% Series
                                 $  per Share of the 4.26% Series
                                 $  per Share of the 4.42% Series
                                 $  per Share of the 4.70% Series
                                 $  per Share of the 7.75% Series
 
   
                                 In addition to the applicable Purchase Price,
                                 Preferred Shareholders will receive accrued and
                                 unpaid dividends for the Shares tendered.
    
 
Independent Offer.............   The Offer for one Series of Preferred is
                                 independent of the Offer for any other Series
                                 of Preferred. The Offer is not conditioned upon
                                 any minimum number of Shares of the applicable
                                 Series of Preferred being tendered. Preferred
                                 Shareholders who wish to tender their Shares
                                 are not required to vote in favor of the
                                 Proposed Amendment. The Offer is subject,
                                 however, to adoption of the Proposed Amend-
 
                                        4
<PAGE>   5
 
                                 ment at the Special Meeting and certain other
                                 conditions described herein. See "Terms of the
                                 Offer -- Certain Conditions of the Offer."
 
Expiration Date of the
Offer.........................   The Offer expires at 5:00 p.m., New York City
                                 time on [Expiration Date], unless extended (the
                                 "Expiration Date").
 
How to Tender Shares..........   See "Terms of the Offer -- Procedure for
                                 Tendering Shares." For further information,
                                 call the Information Agent or the Dealer
                                 Manager or consult your broker for assistance.
 
Withdrawal Rights.............   Tendered Shares of any Series of Preferred may
                                 be withdrawn at any time until the Expiration
                                 Date with respect to such Series of Preferred
                                 and, unless previously accepted for payment,
                                 may also be withdrawn after
                                                     , 1998. See "Terms of the
                                 Offer -- Withdrawal Rights." Any proxy
                                 accompanying any tendered Shares that are
                                 withdrawn will not be considered revoked unless
                                 the Preferred Shareholder specifically revokes
                                 such proxy as described herein. See "Proposed
                                 Amendment and Proxy Solicitation -- Proxies."
 
   
Purpose of the Offer..........   ILN is making the Offer because ILN believes
                                 that the purchase of Shares is economically
                                 attractive to ILN, its shareholders and IPC.
                                 ILN believes the Offer with respect to each
                                 Series of Preferred is fair to unaffiliated
                                 Preferred Shareholders. In making this
                                 determination for each Series of Preferred, ILN
                                 considered that (i) the Offer gives all
                                 Preferred Shareholders the opportunity to sell
                                 Shares of each Series of Preferred at a price
                                 which ILN believes to be a premium over the
                                 respective market price of Shares of each
                                 Series of Preferred on the date of the
                                 announcement of the Offer and (ii) the Offer
                                 provides Preferred Shareholders the opportunity
                                 to sell Shares of each Series of Preferred
                                 without the usual transaction costs associated
                                 with open-market sales. In addition, the Offer
                                 will provide Preferred Shareholders who are not
                                 in favor of the Proposed Amendment the ability
                                 to liquidate their position in the Shares. See
                                 "Special Factors."
    
 
   
Dividends.....................   IPC has declared the regular quarterly dividend
                                 on each Series of Preferred to be paid on May
                                 1, 1998 (the "May 1998 Dividend") to holders of
                                 record as of the close of business on April 9,
                                 1998. A tender and purchase of Shares pursuant
                                 to the Offer will not deprive a Preferred
                                 Shareholder of his or her right to receive the
                                 May 1998 Dividend on his or her Shares held of
                                 record as of the close of business on April 9,
                                 1998. Tendering Preferred Shareholders will be
                                 entitled to any dividends accrued and unpaid
                                 prior to, but not including, the Payment Date
                                 in respect of any later dividend periods (or
                                 any portion thereof).
    
 
Brokerage Commissions.........   Not payable by Preferred Shareholders.
 
   
Solicitation Fee..............   ILN will pay to each designated Soliciting
                                 Dealer (as defined herein) a solicitation fee
                                 of $            per Share that is tendered,
                                 accepted for payment and paid for pursuant to
                                 the Offer in transactions for beneficial owners
                                 of fewer than        Shares and a solicitation
                                 fee of $            per Share for transactions
                                 for beneficial owners of        or more Shares;
                                 provided that solicitation fees payable in
                                 transactions for beneficial owners of        or
    
 
                                        5
<PAGE>   6
 
   
                                 more Shares shall be paid 80% to the Dealer
                                 Manager and 20% to any designated Soliciting
                                 Dealer (which may be the Dealer Manager). In
                                 addition, if the Proposed Amendment is adopted
                                 at the Special Meeting, IPC will pay to each
                                 designated Soliciting Dealer a separate fee of
                                 $            per Share for Shares that are not
                                 tendered pursuant to the Offer but which are
                                 voted in favor of the Proposed Amendment in
                                 transactions for beneficial owners of fewer
                                 than        Shares and a separate fee of
                                 $            per Share for Shares that are not
                                 tendered pursuant to the Offer but which are
                                 voted in favor of the Proposed Amendment in
                                 transactions for beneficial owners of        or
                                 more Shares; provided that the separate fee
                                 payable in transactions for beneficial owners
                                 of        or more Shares shall be paid 80% to
                                 the Dealer Manager and 20% to any designated
                                 Soliciting Dealer (which may be the Dealer
                                 Manager). A Soliciting Dealer will not be
                                 entitled to a solicitation fee or a separate
                                 fee for Shares beneficially owned by such
                                 Soliciting Dealer.
    
 
Proposed Amendment............   Concurrently with the Offer, the Board of
                                 Directors of IPC is soliciting proxies for use
                                 at the Special Meeting. The purpose of the
                                 Special Meeting is to consider an amendment to
                                 IPC's Articles which would remove a provision
                                 that limits IPC's ability to issue or assume
                                 unsecured indebtedness. If the Proposed
                                 Amendment is adopted at the Special Meeting,
                                 the clause of the Articles that places
                                 restrictions on IPC's ability to issue or
                                 assume unsecured indebtedness will be
                                 eliminated with respect to any Shares that
                                 remain outstanding after the consummation of
                                 the Offer. See "Special Factors." Preferred
                                 Shareholders may be entitled to dissenters'
                                 rights in connection with the adoption of the
                                 Proposed Amendment. See "Proposed Amendment and
                                 Proxy Solicitation -- Dissenters' Rights."
 
Special Meeting...............   IPC will consider the Proposed Amendment at the
                                 Special Meeting to be held at its principal
                                 office, 500 South 27th Street, Decatur,
                                 Illinois 62525, on [EXPIRATION DATE] at 4:30
                                 p.m., New York City time. Only holders of
                                 record of IPC's voting securities at the close
                                 of business on the Record Date will be entitled
                                 to vote at the Special Meeting.
 
   
Record Date...................   [RECORD DATE] (the "Record Date").
    
 
   
Special Cash Payment..........   If the Proposed Amendment is adopted at the
                                 Special Meeting by IPC's shareholders, IPC will
                                 make a special cash payment of $      per Share
                                 to each Preferred Shareholder of record as of
                                 the Record Date who did not tender his or her
                                 Shares in the Offer (the "Special Cash
                                 Payment"). Preferred Shareholders who validly
                                 tender their Shares will be entitled only to
                                 the applicable purchase price per Share listed
                                 on the front cover of this Offer to Purchase
                                 and Proxy Statement plus accrued and unpaid
                                 dividends for the Shares tendered.
    
 
Payment Date..................   Promptly after the Expiration Date or any
                                 extension thereof (the "Payment Date").
 
Stock Transfer Tax............   Except as described herein, ILN will pay or
                                 cause to be paid any stock transfer taxes with
                                 respect to the sale and transfer of any Shares
                                        6
<PAGE>   7
 
                                 to it or its order pursuant to the Offer. Each
                                 Preferred Shareholder will be responsible for
                                 paying any income or gross receipts taxes
                                 imposed by any jurisdiction by reason of the
                                 Special Cash Payment and/or the sale of the
                                 Shares. See Instruction 6 of the applicable
                                 Letter of Transmittal and Proxy. See also
                                 "Terms of the Offer -- Acceptance of Shares for
                                 Payment and Payment of Purchase Price" and
                                 "Certain Federal Income Tax Consequences."
 
   
Further Information...........   Additional copies of this Offer to Purchase and
                                 Proxy Statement, the applicable Letter of
                                 Transmittal and Proxy, and the applicable
                                 Separate Proxy may be obtained by contacting
                                 MacKenzie Partners, Inc., at (800) 322-2885
                                 (toll free) or (212) 929-5500 (brokers and
                                 dealers). Questions about the Offer should be
                                 directed to DLJ at (800) 334-1604 (toll free).
    
 
                                        7
<PAGE>   8
 
                                SPECIAL FACTORS
 
PURPOSE OF THE OFFER; CERTAIN EFFECTS OF THE OFFER
 
   
     ILN believes that the purchase of the Shares at this time represents an
attractive economic opportunity that will benefit ILN, its shareholders and IPC.
ILN believes the Offer with respect to each Series of Preferred is fair to
unaffiliated Preferred Shareholders. In making this determination for each
Series of Preferred, ILN considered that (i) the Offer gives all Preferred
Shareholders the opportunity to sell Shares of each Series of Preferred at a
price which ILN believes to be a premium over the respective market price of
Shares of each Series of Preferred on the date of the announcement of the Offer
and (ii) the Offer provides Preferred Shareholders the opportunity to sell
Shares of each Series of Preferred without the usual transaction costs
associated with open-market sales. In addition, the Offer will provide Preferred
Shareholders who are not in favor of the Proposed Amendment the ability to
liquidate their position in the Shares. ILN did not find it practicable to, and
did not, quantify or otherwise assign relative weights to these considerations.
Trading of the Shares of each Series of Preferred has been limited and sporadic.
ILN determined the Purchase Price for each Series of Preferred with reference to
certain objective factors, including the relationship of prices offered by
utilities for preferred stock with dividend characteristics similar to the
respective Series of Preferred to prevailing yields on reference securities such
as U.S. Treasury Bonds and other municipal securities at the time. ILN also
considered the historical and recent market prices of each Series of Preferred
and the current and future redemption prices of each Series of Preferred. In
addition to these objective factors, ILN took into account certain subjective
factors, including general industry outlook, general market supply of securities
of similar type, and supply and demand factors in the securities markets
generally. Although the weight ILN placed on these factors was subjective, ILN
gave relatively more weight to the objective factors described above. ILN does
not believe that the transaction will result in a material change to IPC's net
book value, going concern value or future prospects and, accordingly, ILN did
not take these factors into account in determining the fairness of such Purchase
Price to unaffiliated Preferred Shareholders. Moreover, ILN did not consider the
liquidation value of any of the Shares or the prices paid in prior purchases by
IPC of the Shares in determining the Purchase Price for any Series of Preferred
as it does not believe that these factors are relevant or meaningful in such
determination. Preferred Shareholders should understand that valuation
methodologies other than the methodology used by ILN to determine the Purchase
Prices for each Series of Preferred might produce values for such Series of
Preferred which are greater or less than the Purchase Prices offered by ILN.
    
 
   
     The following table shows, with respect to each Series of Preferred the
Purchase Price offered by ILN, the last reported sale price on the NYSE (or, in
the case of the 7.75% Series, the OTC) prior to the date of commencement of the
Offer on April   , 1998 and the resulting premium represented by the Purchase
Price (after subtracting the applicable accrued dividend component of the May
1998 Dividend from the last reported sale price):
    
 
   
<TABLE>
<CAPTION>
          SERIES OF             PURCHASE PRICE      LAST REPORTED      PREMIUM REPRESENTED BY
          PREFERRED               PER SHARE       SALE PRICE (DATE)      THE PURCHASE PRICE
          ---------             --------------    -----------------    ----------------------
<S>                             <C>               <C>                  <C>
4.08%.........................
4.20%.........................
4.26%.........................
4.42%.........................
4.70%.........................
7.75%.........................
</TABLE>
    
 
   
     ILN established the premium levels for the Series of Preferred at levels
that ILN believes are, together with the elimination of transaction costs,
attractive to the Preferred Shareholders based upon a comparison with premiums
offered by utilities for preferred stock for which there is an established
trading market, while at the same time preserving for ILN the economic appeal of
the Offer. Given the somewhat limited and sporadic trading of the Shares,
particularly the Shares of the 7.75% Series, ILN is not able to determine
conclusively whether the Purchase Price for each Series of Preferred represents
a premium over the price that could otherwise be obtained.
    
 
                                        8
<PAGE>   9
 
     Neither ILN, IPC nor their respective boards of directors (individually, a
"Board of Directors" and collectively, the "Boards of Directors") received any
report, opinion or appraisal from an outside party which is related to the
Offer, including, but not limited to, any report, opinion or appraisal relating
to the consideration or the fairness of the consideration to be offered to the
holders of the Shares or the fairness of such Offer to ILN, IPC or the
unaffiliated holders of Shares. Neither of the Boards of Directors nor any
individual director of ILN or IPC has retained an unaffiliated representative to
act solely on behalf of unaffiliated holders of Shares for the purposes of
negotiating the terms of the Offer or preparing a report concerning the fairness
of the Offer. Neither ILN, IPC nor their respective Boards of Directors believed
these measures were necessary to ensure fairness in light of the fact that the
Offer will not result in a liquidation or change in control in ILN, IPC or any
of their affiliates.
 
     Preferred Shareholders are not under any obligation to tender Shares
pursuant to the Offer. The Offer does not constitute notice of redemption of any
Series of Preferred pursuant to IPC's Articles, nor does ILN or IPC intend to
effect any such redemption by making the Offer. Further, the Offer does not
constitute a waiver by IPC of any option it has to redeem Shares. The 4.08%
Series, 4.20% Series, 4.26% Series, 4.42% Series and 4.70% Series of cumulative
preferred stock are not subject to mandatory redemption, but presently are
redeemable at the option of IPC at $51.50 per Share, $52.00 per Share, $51.50
per Share, $51.50 per Share and $51.50 per Share respectively. The 7.75% Series
is also not subject to mandatory redemption and is not redeemable until July 1,
2003. On or after July 1, 2003, the 7.75% Series is redeemable at the option of
IPC at $50.00 per Share. The Shares of each Series of Preferred have no
preemptive or conversion rights. Upon liquidation or dissolution of IPC, owners
of the Shares would be entitled to receive an amount equal to the liquidation
preference of $50.00 per share plus all accrued and unpaid dividends (whether or
not earned or declared) thereon to the date of payment, prior to the payment of
any amounts to the holders of IPC's common stock (currently, ILN is the only
holder of IPC's common stock).
 
   
     Shares validly tendered to the Depositary pursuant to the Offer and not
withdrawn in accordance with the procedures set forth herein shall be held until
the Expiration Date (or returned to the extent the Offer is terminated in
accordance herewith). To the extent the Proposed Amendment is adopted at the
Special Meeting and the Shares tendered are accepted for payment and paid for in
accordance with the terms hereof, ILN may at any time thereafter sell or
otherwise transfer Shares to IPC and, at that time, it is expected that IPC
would retire and cancel the Shares. However, in the event the Proposed Amendment
is not adopted at the Special Meeting, ILN may elect to, but is not obligated
to, waive, subject to applicable law, such condition. In that case, subsequent
to ILN's waiver and purchase of the Shares, IPC anticipates, as promptly as
practicable thereafter, that it would call another special meeting of its
shareholders and solicit proxies therefrom for an amendment substantially
similar to the Proposed Amendment. At that meeting, ILN would vote any Shares
purchased by it pursuant to the Offer or otherwise (together with its shares of
IPC's common stock) in favor of such amendment, thereby maximizing the prospects
for the adoption of the amendment.
    
 
   
     The purchase of Shares of each Series of Preferred pursuant to the Offer
will reduce the number of holders of Shares and the number of such Shares that
might otherwise trade publicly, and, depending upon the number of Shares so
purchased, such reduction could adversely affect the ratings, liquidity and
market value of the remaining Shares of each such Series of Preferred held by
the public. The extent of the public market for such Shares and the availability
of price quotations would, however, depend upon such factors as the number of
shareholders remaining at such time, the interest in maintaining a market in the
Shares on the part of securities firms and other factors. As of the Record Date,
there were           registered holders of the 4.08% Series,
registered holders of the 4.20% Series,           registered holders of the
4.26% Series,           registered holders of the 4.42% Series,
registered holders of the 4.70% Series and           registered holders of the
7.75% Series. Depending on the number of Shares tendered and purchased pursuant
to the Offer, the Series of Preferred listed and trading on the NYSE may no
longer meet the requirements of the NYSE for continued listing, which could
adversely affect the market for such Shares. In addition, each Series of
Preferred (other than the 7.75% Series) is currently registered under Section
12(b) of the Securities Exchange Act of 1934, as amended (the "Exchange Act").
Registration of such Shares under the Exchange Act may be terminated upon the
application by IPC to the SEC if such Shares are neither listed on a national
securities exchange nor held by more than 300 holders of record. Termination of
registration of
    
 
                                        9
<PAGE>   10
 
such Shares under the Exchange Act would substantially reduce the information
required to be furnished to Preferred Shareholders and could make certain
provisions of the Exchange Act, such as the requirement of Rule 13e-3 thereunder
with respect to "going private transactions," no longer applicable to IPC.
 
   
     As of the date of this Offer to Purchase and Proxy Statement, each Series
of Preferred was rated Baa2, BBB- and BBB-, by Moody's Investors Service, Inc.
("Moody's"), Standard & Poor's Ratings Services, a division of The McGraw-Hill
Companies, Inc. ("S&P") and Duff & Phelps Credit Rating Company ("D&P"),
respectively.
    
 
LIQUIDITY OF TRADING MARKET
 
     To the extent that Shares of any Series of Preferred are tendered, accepted
for payment and paid for in the Offer, the trading market for Shares of such
Series of Preferred that remain outstanding may be significantly more limited,
which might adversely affect the liquidity, market value and price volatility of
such Shares. Equity securities with a smaller outstanding market value available
for trading (the "float") may command a lower price than would comparable equity
securities with a greater float. Therefore, the market price for Shares that are
not tendered in the Offer may be affected adversely to the extent that the
number of Shares purchased pursuant to the Offer reduces the float. A reduced
float may also make the trading price of the Shares that are not tendered,
accepted for payment and paid for in the Offer more volatile. Preferred
Shareholders of the remaining Shares may attempt to obtain quotations for the
Shares from their brokers; however, there can be no assurance that any trading
market will exist for such Shares following consummation of the Offer. To the
extent a market continues to exist for the Shares after the Offer, the Shares
may trade at a discount compared to present trading depending on the market for
shares with similar features, the performance of IPC and other factors. There is
no assurance that an active market in the Shares will exist and no assurance as
to the prices at which the Shares may trade.
 
POTENTIAL EFFECTS OF THE PROPOSED AMENDMENT ON NON-TENDERING PREFERRED
SHAREHOLDERS
 
     If the Proposed Amendment is adopted at the Special Meeting, Preferred
Shareholders of Shares that are not tendered and purchased pursuant to the Offer
will no longer be entitled to the benefits of the Debt Limitation Provision,
which will have been removed by the Proposed Amendment. As discussed above, the
Debt Limitation Provision places restrictions on IPC's ability to issue or
assume unsecured indebtedness. Although IPC's debt instruments may contain
certain restrictions on IPC's ability to issue or assume debt, any such
restrictions may be waived and the increased flexibility afforded IPC by the
removal of the Debt Limitation Provision may permit IPC to take certain actions
that may increase the credit risks with respect to IPC, adversely affecting the
market price and credit rating of the remaining Shares or otherwise be
materially adverse to the interests of the remaining Preferred Shareholders. In
addition, to the extent that IPC elects to issue or assume additional unsecured
indebtedness, the remaining Preferred Shareholders' relative position in IPC's
capital structure could be perceived to decline, which in turn could adversely
affect the market prices and credit ratings of the remaining Shares.
 
PLANS OF ILN AND IPC AFTER THE OFFER
 
     Following the consummation of the Offer, the business and operations of IPC
are expected to be continued substantially as they are currently being
conducted. Except as disclosed herein, ILN and IPC currently have no plans or
proposals that relate to or would result in: (i) the acquisition by any person
or entity of additional securities of IPC or the disposition of securities of
IPC, other than in the ordinary course of business; (ii) an extraordinary
corporate transaction, such as a merger, reorganization or liquidation,
involving IPC or any of its subsidiaries; (iii) a sale or transfer of a material
amount of assets of IPC or any of its subsidiaries; (iv) any change in the
present Board of Directors or management of IPC; (v) any material change in the
present dividend rate or policy, or indebtedness or capitalization of IPC; (vi)
any other material change in IPC's corporate structure or business; (vii) any
change in IPC's Articles or by-laws ("By-laws") or any actions that may impede
the acquisition of control of IPC by any person; (viii) a class of equity
securities of IPC being delisted from a national securities exchange or no
longer authorized to be quoted on the OTC; (ix) a class of equity securities of
IPC becoming eligible for termination of registration pursuant to
                                       10
<PAGE>   11
 
   
Section 12(g)(4) of the Exchange Act; or (x) the suspension of IPC's obligation
to file reports pursuant to Section 15(d) of the Exchange Act. However, ILN and
IPC have from time to time considered, and expect to consider in the future,
various strategies designed to enhance IPC's ability to anticipate and adapt to
changes in the increasingly competitive utility industry. ILN and IPC are
currently developing strategies responsive to the enactment of Public Act 90-561
("PA 90-561") in Illinois on December 16, 1997. PA 90-561 amends the Illinois
Public Utilities Act in numerous respects and provides a comprehensive framework
for restructuring the electric utility industry in Illinois, including a managed
transition to a competitive industry. PA 90-561 permits electric utilities in
Illinois, including IPC, upon approval of the Illinois Commerce Commission
("ICC"), to issue, directly or indirectly, securitization bonds on or after
August 1, 1998 in an aggregate amount not to exceed, among certain other
restrictions, 50% of the electric utility's total capitalization as of December
31, 1996, with the proceeds of such issuance generally limited to the
refinancing of debt or equity of the electric utility. IPC has filed an
application with the ICC seeking the approvals required for such issuance of
securitization bonds and may determine to proceed with such issuance in 1998. In
addition, ILN and IPC may pursue other business strategies, including business
combinations with other companies, internal restructurings involving the
complete or partial separation of IPC's generation, transmission and
distribution businesses, acquisitions or dispositions of assets or lines of
business, and additions to or reductions of franchised service territories. ILN
and IPC may from time to time engage in discussions, either internally or with
third parties, regarding one or more of these potential strategies. Those
discussions may be subject to confidentiality agreements and the policy of ILN
and IPC is generally not to comment on such activities. No assurances can be
given that any potential transaction of the types described above may actually
occur, or if one does occur, no assurances can be given with respect to the
ultimate effect of any such transaction on ILN's or IPC's results or operations,
financial condition or competitive position.
    
 
     After the consummation of the Offer, ILN or IPC, in its sole discretion,
may purchase additional Shares on the open market, in privately negotiated
transactions, through one or more tender offers or otherwise. Any such purchases
may be on the same terms as, or on terms which are more or less favorable to
holders of Shares than, the terms of the Offer. However, Rule 13e-4(f)(6) under
the Exchange Act, prohibits ILN and its affiliates (including IPC) from
purchasing any Shares of a Series of Preferred, other than pursuant to the
Offer, until at least ten business days after the Expiration Date with respect
to that Series of Preferred. Any future purchases of Shares by ILN or IPC would
depend on many factors, including the success of the Offer, the market price of
the Shares, ILN's business and financial position, legal restrictions on ILN's
ability to purchase Shares as well as general economic and market conditions.
 
     NEITHER ILN, IPC, THEIR RESPECTIVE BOARDS OF DIRECTORS, NOR ANY OF THEIR
RESPECTIVE OFFICERS MAKES ANY RECOMMENDATION TO ANY PREFERRED SHAREHOLDER AS TO
WHETHER TO TENDER ANY OR ALL SHARES. EACH PREFERRED SHAREHOLDER MUST MAKE HIS OR
HER OWN DECISION AS TO WHETHER TO TENDER SHARES AND, IF SO, HOW MANY SHARES TO
TENDER.
 
CERTAIN LEGAL MATTERS; REGULATORY APPROVALS
 
   
     Neither ILN nor IPC needs to obtain approval from the ICC to undertake the
Offer or the proxy solicitation. ILN is not aware of any license or regulatory
permit that would be material to ILN's or IPC's business that might be adversely
affected by ILN's acquisition of Shares as contemplated in the Offer or of any
other approval or other action by any government or governmental, administrative
or regulatory authority or agency, domestic or foreign, that would be required
for ILN's acquisition of Shares pursuant to the Offer. Should any approval or
other action be required, ILN currently contemplates that it will seek such
approval or other action. ILN cannot predict whether it could be required to
delay the acceptance for payment of, or payment for, Shares tendered pursuant to
the Offer pending the outcome of any such matter. There can be no assurance that
any such approval or other action, if needed, would be obtained, or would be
obtained without substantial conditions or that the failure to obtain any such
approval or other action might not result in adverse consequences to ILN's or
IPC's business. ILN and IPC intend to make all required filings under the
Exchange Act. ILN's obligation under the Offer to accept for payment, or make
payment for, Shares is subject to certain conditions. See "Terms of the Offer --
Certain Conditions of the Offer."
    
 
                                       11
<PAGE>   12
 
     Except for adoption of the Proposed Amendment, which condition can be
waived by ILN, no approval of the holders of any Shares or the holders of any of
IPC's other securities or the holders of ILN's securities is required in
connection with the Offer. See "Proposed Amendment and Proxy Solicitation."
 
DISSENTERS' RIGHTS
 
     No dissenters' rights are available to holders of Shares in connection with
the Offer. However, holders of Shares may be entitled to dissenters' rights in
connection with the adoption of the Proposed Amendment. See "Proposed Amendment
and Proxy Solicitation -- Dissenters' Rights."
 
                               TERMS OF THE OFFER
 
NUMBER OF SHARES; PURCHASE PRICES; EXPIRATION DATE; DIVIDENDS
 
   
     Upon the terms and subject to the conditions described herein and in the
applicable Letter of Transmittal and Proxy, ILN will purchase any and all Shares
that are validly tendered on or prior to the Expiration Date (and not properly
withdrawn in accordance with "Terms of the Offer -- Withdrawal Rights") at the
purchase price per Share listed on the front cover of this Offer to Purchase and
Proxy Statement plus accrued and unpaid dividends for the Shares tendered, net
to the seller in cash. See "Terms of the Offer -- Certain Conditions of the
Offer" and "Terms of the Offer -- Extension of Tender Period; Termination;
Amendments." On the Record Date, there were issued and outstanding 283,290
Shares of the 4.08% Series of Preferred; 167,720 Shares of the 4.20% Series of
Preferred; 136,000 Shares of the 4.26% Series of Preferred; 134,400 Shares of
the 4.42% Series of Preferred; 176,000 Shares of the 4.70% Series of Preferred;
and 241,700 Shares of the 7.75% Series of Preferred.
    
 
     THE OFFER FOR A SERIES OF PREFERRED IS NOT CONDITIONED UPON ANY MINIMUM
NUMBER OF SHARES OF SUCH SERIES OF PREFERRED BEING TENDERED AND IS INDEPENDENT
OF THE OFFER FOR ANY OTHER SERIES OF PREFERRED. THE OFFER, HOWEVER, IS
CONDITIONED UPON, AMONG OTHER THINGS, THE PROPOSED AMENDMENT, AS DESCRIBED
HEREIN, BEING ADOPTED AT THE SPECIAL MEETING. SEE "TERMS OF THE OFFER -- CERTAIN
CONDITIONS OF THE OFFER."
 
   
     The Offer is being sent to all persons in whose names Shares are registered
on the books of IPC as of the close of business on the Record Date and
transferees of such persons. Only a record holder of Shares on the Record Date
may vote in person or by proxy at the Special Meeting. No record date is fixed
for determining which persons are permitted to tender Shares. Any person who is
the beneficial owner but not the record holder of Shares must arrange for the
record transfer of such Shares prior to tendering.
    
 
   
     With respect to each Series of Preferred, the Expiration Date is the later
of 5:00 p.m., New York City time, on [EXPIRATION DATE] or the latest time and
date to which the Offer with respect to such Series of Preferred is extended.
ILN expressly reserves the right, in its sole discretion, and at any time and/or
from time to time, to extend the period of time during which the Offer for any
Series of Preferred is open, by giving oral or written notice of such extension
to the Depositary and making a public announcement thereof, without extending
the period of time during which the Offer for any other Series of Preferred is
open. There is no assurance whatsoever that ILN will exercise its right to
extend the Offer for any Series of Preferred. If ILN decides, in its sole
discretion, to (i) decrease the number of Shares of any Series of Preferred
being sought, (ii) increase or decrease the consideration offered in the Offer
to holders of any Series of Preferred or (iii) increase or decrease the
Soliciting Dealers' fees and, at the time that notice of such increase or
decrease is first published, sent or given to holders of such Series of
Preferred in the manner specified herein, the Offer for such Series of Preferred
is scheduled to expire at any time earlier than the tenth business day from the
date that such notice is first so published, sent or given, such Offer will be
extended until the expiration of such ten-business-day period. For purposes of
the Offer, a "business day" means any day other than a Saturday, Sunday or
federal holiday and consists of the time period from 12:01 a.m. through 12:00
midnight, New York City time.
    
 
                                       12
<PAGE>   13
 
      NO ALTERNATIVE, CONDITIONAL OR CONTINGENT TENDERS WILL BE ACCEPTED.
 
   
     The May 1998 Dividend has been declared on each Series of Preferred,
payable May 1, 1998 to holders of record as of the close of business on April 9,
1998. A tender and purchase of Shares pursuant to the Offer will not deprive
such Preferred Shareholder of his or her right to receive the May 1998 Dividend
on his or her Shares held of record on April 9, 1998, regardless of when such
tender is made. Tendering Preferred Shareholders will be entitled to any
dividends accrued and unpaid prior to, but not including, the Payment Date in
respect of any later dividend periods (or any portion thereof). The payment of
the May 1998 Dividend will be made separately from payments for Shares tendered
in the Offer or the payment of any Special Cash Payments.
    
 
PROCEDURE FOR TENDERING SHARES
 
     To tender Shares pursuant to the Offer, the tendering owner of Shares must
either:
 
   
          (i) send to the Depositary (at one of its addresses set forth on the
     back cover of this Offer to Purchase and Proxy Statement) a properly
     completed and duly executed Letter of Transmittal and Proxy (or in the case
     of an Eligible Institution, a facsimile thereof) together with any required
     signature guarantees and any other documents required by the Letter of
     Transmittal and Proxy and either (a) certificates for the Shares to be
     tendered must be received by the Depositary at one of such addresses or (b)
     such Shares must be delivered pursuant to the procedures for book-entry
     transfer described herein (and a confirmation of such delivery must be
     received by the Depositary), in each case by 5:00 p.m., New York City time,
     on the Expiration Date; or
    
 
          (ii) comply with the guaranteed delivery procedure described under
     "Guaranteed Delivery Procedure" below.
 
A tender of Shares made pursuant to any method of delivery set forth herein or
in the Letter of Transmittal and Proxy will constitute a binding agreement
between the tendering holder and ILN upon the terms and subject to the
conditions of the Offer.
 
     The Depositary will establish an account with respect to the Shares at The
Depository Trust Company (the "Book-Entry Transfer Facility") for purposes of
the Offer within two business days after the date of this Offer to Purchase and
Proxy Statement, and any financial institution that is a participant in the
system of a Book-Entry Transfer Facility may make delivery of Shares by causing
the Book-Entry Transfer Facility to transfer such Shares into the Depositary's
account in accordance with the procedures of the Book-Entry Transfer Facility.
Although delivery of Shares may be effected through book-entry transfer, such
delivery must be accompanied by either (i) a properly completed and duly
executed Letter of Transmittal and Proxy, together with any required signature
guarantees and any other required documents or (ii) an Agent's Message (as
hereinafter defined) and, in any case, must be received by the Depositary at one
of its addresses set forth on the back cover of this Offer to Purchase and Proxy
Statement on or prior to the Expiration Date. The confirmation of a book entry
transfer of Shares into the Depositary's account at the Book-Entry Transfer
Facility as described above is referred to as a "Book-Entry Confirmation."
 
     DELIVERY OF THE LETTER OF TRANSMITTAL AND PROXY AND ANY OTHER REQUIRED
DOCUMENTS TO THE BOOK-ENTRY TRANSFER FACILITY OR TO ILN DOES NOT CONSTITUTE
DELIVERY TO THE DEPOSITARY.
 
     The term "Agent's Message" means a message, transmitted by the Book-Entry
Transfer Facility, received by the Depositary and forming a part of the
book-entry transfer when a tender is initiated, which states that the Book-Entry
Transfer Facility has received an express acknowledgment from a participant
tendering Shares, that such participant has received and agrees to be bound by
the terms of the Letter of Transmittal and Proxy and that ILN may enforce such
agreement against such participant.
 
     Signature Guarantees. Except as otherwise provided below, all signatures on
a Letter of Transmittal and Proxy must be guaranteed by a firm that is a member
in good standing of a registered national securities exchange or the National
Association of Securities Dealers, Inc. ("NASD"), or by a commercial bank or
trust company having an office or correspondent in the United States that is a
participant in an approved
 
                                       13
<PAGE>   14
 
Signature Guarantee Medallion Program (each of the foregoing being referred to
as an "Eligible Institution"). Signatures on a Letter of Transmittal and Proxy
need not be guaranteed if (i) the Letter of Transmittal and Proxy is signed by
the registered owner of the Shares tendered therewith and such owner has not
completed the box entitled "Special Payment Instructions" or the box entitled
"Special Delivery Instructions" on the Letter of Transmittal and Proxy or (ii)
such Shares are tendered for the account of an Eligible Institution. See
Instructions 1 and 5 of the Letter of Transmittal and Proxy. If Shares are
registered in the name of a person other than the signatory on the Letter of
Transmittal and Proxy, or if unpurchased Shares are to be issued to a person
other than the registered holder(s), the certificates must be endorsed or
accompanied by appropriate stock powers, in either case signed exactly as the
name or names of the registered holder(s) appear on the Shares with the
signature(s) on the Shares or stock powers guaranteed as stated above. See
Instructions 4, 6 and 7 to the Letter of Transmittal and Proxy.
 
     Guaranteed Delivery Procedure. If a Preferred Shareholder desires to tender
Shares pursuant to the Offer and such Preferred Shareholder's certificates are
not immediately available or the procedures for book-entry transfer cannot be
completed on a timely basis or time will not permit all required documents to
reach the Depositary prior to the Expiration Date, such Shares may nevertheless
be tendered if all of the following guaranteed delivery procedures are complied
with:
 
          (i) such tender is made by or through an Eligible Institution;
 
          (ii) a properly completed and duly executed Notice of Guaranteed
     Delivery, substantially in the form provided by ILN and IPC herewith, is
     received (with any required signatures or signature guarantees) by the
     Depositary as provided below on or prior to 5:00 p.m., New York City time,
     on the Expiration Date; and
 
          (iii) the certificates for all tendered Shares in proper form for
     transfer or a Book-Entry Confirmation with respect to all tendered Shares,
     together with a properly completed and duly executed Letter of Transmittal
     and Proxy, or a manually signed facsimile thereof, and any other documents
     required by the Letter of Transmittal and Proxy, are received by the
     Depositary no later than 5:00 p.m., New York City time, within three
     business days after the date of execution of such Notice of Guaranteed
     Delivery.
 
     THE NOTICE OF GUARANTEED DELIVERY MAY BE DELIVERED BY HAND OR MAILED OR
TRANSMITTED BY FACSIMILE TO THE DEPOSITARY AND MUST INCLUDE AN ENDORSEMENT BY AN
ELIGIBLE INSTITUTION IN THE FORM SET FORTH IN SUCH NOTICE OF GUARANTEED
DELIVERY.
 
     In all cases, Shares shall not be deemed validly tendered unless a properly
completed and duly executed Letter of Transmittal and Proxy, a manually signed
facsimile thereof, or, if applicable, an Agent's Message, is received by the
Depositary.
 
     Notwithstanding any other provision hereof, payment for Shares accepted for
payment pursuant to the Offer in all cases will be made only after timely
receipt by the Depositary of certificates for (or an Agent's Message with
respect to) such Shares, a Letter of Transmittal and Proxy, properly completed
and duly executed, or a manually signed facsimile thereof, with any required
signature guarantees and all other documents required by the Letter of
Transmittal and Proxy. Accordingly, tendering Preferred Shareholders may be paid
at different times depending upon when certificates for Shares or Book-Entry
Confirmations are actually received by the Depositary. Under no circumstances
will interest be paid on the Purchase Price for Shares tendered to ILN pursuant
to the Offer, regardless of any extension of the Offer or any delay in making
such payment.
 
     THE METHOD OF DELIVERY OF SHARES AND ALL OTHER REQUIRED DOCUMENTS IS AT THE
OPTION AND RISK OF THE TENDERING PREFERRED SHAREHOLDER. IF DELIVERY IS BY MAIL,
REGISTERED MAIL WITH RETURN RECEIPT REQUESTED, PROPERLY INSURED, IS RECOMMENDED.
BECAUSE IT IS THE TIME OF RECEIPT, NOT THE TIME OF MAILING, WHICH DETERMINES
WHETHER A TENDER HAS BEEN MADE PRIOR TO THE EXPIRATION DATE, SUFFICIENT TIME
SHOULD BE ALLOWED TO ASSURE TIMELY DELIVERY.
 
                                       14
<PAGE>   15
 
     TO AVOID FEDERAL INCOME TAX BACKUP WITHHOLDING EQUAL TO 31% OF THE GROSS
PAYMENTS MADE PURSUANT TO THE OFFER, EACH TENDERING PREFERRED SHAREHOLDER WHO IS
A UNITED STATES PERSON MUST NOTIFY THE DEPOSITARY OF SUCH PREFERRED
SHAREHOLDER'S CORRECT TAXPAYER IDENTIFICATION NUMBER AND PROVIDE CERTAIN OTHER
INFORMATION BY PROPERLY COMPLETING AND EXECUTING THE SUBSTITUTE FORM W-9
INCLUDED IN THE LETTER OF TRANSMITTAL AND PROXY OR IN THE SEPARATE PROXY (OR, IN
THE CASE OF A FOREIGN PREFERRED SHAREHOLDER, FORM W-8 OBTAINABLE FROM THE
DEPOSITARY). SEE "CERTAIN FEDERAL INCOME TAX CONSEQUENCES."
 
     EACH PREFERRED SHAREHOLDER IS URGED TO CONSULT WITH HIS OR HER OWN TAX
ADVISOR REGARDING THE TAX CONSEQUENCES OF THE OFFER.
 
     Determinations of Validity. All questions as to the form of documents and
the validity, eligibility (including the time of receipt) and acceptance for
payment of any tender of Shares will be determined by ILN, in its sole
discretion, and its determination will be final and binding. ILN reserves the
absolute right to reject any or all tenders of Shares that (i) it determines are
not in proper form or (ii) the acceptance for payment of or payment for which
may, in the opinion of ILN's counsel, be unlawful. ILN also reserves the
absolute right to waive any defect or irregularity in any tender of Shares. No
tender of Shares will be deemed to be properly made until all defects or
irregularities have been cured or waived. None of ILN, IPC, the Dealer Manager,
the Depositary, the Information Agent or any other person will be under any duty
to give notice of any defect or irregularity in tenders, nor shall any of them
incur any liability for failure to give any such notice.
 
WITHDRAWAL RIGHTS
 
     Tenders of Shares made pursuant to the Offer may be withdrawn at any time
prior to the Expiration Date. Thereafter, such tenders are irrevocable, except
that they may be withdrawn after                     , 1998 unless previously
accepted for payment as provided in this Offer to Purchase and Proxy Statement.
 
     The proxy accompanying any tendered Shares that are withdrawn will not be
considered revoked unless the Preferred Shareholder specifically revokes such
proxy as described herein. See "Proposed Amendment and Proxy Solicitation --
Proxies."
 
     To be effective, a written or facsimile notice of withdrawal must be timely
received by the Depositary, at one of its addresses set forth on the back cover
of this Offer to Purchase and Proxy Statement, and must specify the name of the
person who tendered the Shares to be withdrawn and the number of Shares to be
withdrawn. If the Shares to be withdrawn have been delivered to the Depositary,
a signed notice of withdrawal with signatures guaranteed by an Eligible
Institution (except in the case of Shares tendered by an Eligible Institution)
must be submitted prior to the release of such Shares. In addition, such notice
must specify, in the case of Shares tendered by delivery of certificates, the
name of the registered owner (if different from that of the tendering Preferred
Shareholder) and the serial numbers shown on the particular certificates
evidencing the Shares to be withdrawn or, in the case of Shares tendered by
book-entry transfer, the name and number of the account at the Book-Entry
Transfer Facility to be credited with the withdrawn Shares and the name of the
registered holder (if different from the name of such account). Withdrawals may
not be rescinded, and Shares withdrawn will thereafter be deemed not validly
tendered for purposes of the Offer. However, withdrawn Shares may be re-tendered
by again following one of the procedures described in "Terms of the Offer --
Procedure for Tendering Shares" at any time prior to the Expiration Date.
 
     All questions as to the form and validity (including time of receipt) of
any notice of withdrawal will be determined by ILN, in its sole discretion, and
its determination will be final and binding. None of ILN, IPC, the Dealer
Manager, the Depositary, the Information Agent or any other person will be under
any duty to give notification of any defect or irregularity in any notice of
withdrawal or will incur any liability for failure to give any such
notification.
 
                                       15
<PAGE>   16
 
ACCEPTANCE OF SHARES FOR PAYMENT AND PAYMENT OF PURCHASE PRICE
 
     Upon the terms and subject to the conditions of the Offer, and as promptly
as practicable after the Expiration Date, ILN will accept for payment (and
thereby purchase) and pay for Shares validly tendered and not withdrawn as
permitted in "Terms of the Offer -- Withdrawal Rights." Thereafter, payment for
all Shares validly tendered on or prior to the Expiration Date and accepted
pursuant to the Offer will be made by the Depositary as promptly as practicable
after the Expiration Date. In all cases, payment for Shares accepted for payment
pursuant to the Offer will be made promptly but only after timely receipt by the
Depositary of certificates for such Shares (or of an Agent's Message), a
properly completed and duly executed Letter of Transmittal and Proxy (or in the
case of an Eligible Institution, a facsimile thereof) and any other required
documents. Accordingly, tendering Preferred Shareholders may be paid at
different times depending upon when Share certificates or Book-Entry
Confirmations are actually received by the Depositary.
 
   
     For purposes of the Offer, ILN will be deemed to have accepted for payment
(and thereby purchased) Shares that are validly tendered and not withdrawn as,
if and when it gives oral or written notice to the Depositary of its acceptance
for payment of such Shares. ILN will pay for Shares that it has purchased
pursuant to the Offer by depositing the purchase price therefor plus accrued and
unpaid dividends thereon with the Depositary, which will act as agent for
tendering Preferred Shareholders for the purpose of receiving payment from ILN
and transmitting payment to tendering Preferred Shareholders. UNDER NO
CIRCUMSTANCES WILL INTEREST BE PAID ON AMOUNTS TO BE PAID TO TENDERING PREFERRED
SHAREHOLDERS, REGARDLESS OF ANY DELAY IN MAKING SUCH PAYMENT.
    
 
     Certificates for all Shares not validly tendered will be returned or, in
the case of Shares tendered by book-entry transfer, such Shares will be credited
to an account maintained with the Book-Entry Transfer Facility, as promptly as
practicable, without expense to the tendering Preferred Shareholder.
 
     Payment for Shares may be delayed in the event of difficulty in determining
the number of Shares properly tendered. If certain events occur, ILN may not be
obligated to purchase Shares pursuant to the Offer. See "Terms of the Offer --
Certain Conditions of the Offer."
 
     ILN will pay or cause to be paid any stock transfer taxes with respect to
the sale and transfer of any Shares to it or its order pursuant to the Offer.
If, however, payment of the purchase price is to be made to, or Shares not
tendered or not purchased are to be registered in the name of, any person other
than the registered owner, or if tendered Shares are registered in the name of
any person other than the person signing the Letter of Transmittal and Proxy,
the amount of any stock transfer taxes (whether imposed on the registered owner,
such other person or otherwise) payable on account of the transfer to such
person will be deducted from the purchase price unless satisfactory evidence of
the payment of such taxes, or exemption therefrom, is submitted. Each Preferred
Shareholder will be responsible for paying any income or gross receipts taxes
imposed by any jurisdiction by reason of the sale of the Shares or the Special
Cash Payment, as described herein. See Instruction 6 of the accompanying Letter
of Transmittal and Proxy. See also "Proposed Amendment and Proxy Solicitation --
Special Cash Payments" and "Certain Federal Income Tax Consequences."
 
CERTAIN CONDITIONS OF THE OFFER
 
   
     ILN WILL NOT BE REQUIRED TO ACCEPT FOR PAYMENT OR PAY FOR ANY SHARES
TENDERED IF THE PROPOSED AMENDMENT IS NOT ADOPTED AT THE SPECIAL MEETING.
PREFERRED SHAREHOLDERS OF RECORD HAVE THE RIGHT TO VOTE FOR THE PROPOSED
AMENDMENT REGARDLESS OF WHETHER THEY TENDER THEIR SHARES. IF THE PROPOSED
AMENDMENT IS ADOPTED AT THE SPECIAL MEETING, IPC WILL MAKE A SPECIAL CASH
PAYMENT TO EACH PREFERRED SHAREHOLDER OF RECORD AS OF THE RECORD DATE WHO DID
NOT TENDER HIS OR HER SHARES IN THE OFFER. PREFERRED SHAREHOLDERS WHO TENDER
THEIR SHARES WILL BE ENTITLED ONLY TO THE APPLICABLE PURCHASE PRICE PER SHARE
LISTED ON THE FRONT COVER OF THIS OFFER TO PURCHASE AND PROXY STATEMENT PLUS
ACCRUED AND UNPAID DIVIDENDS FOR THE SHARES TENDERED.
    
                                       16
<PAGE>   17
 
     In addition, notwithstanding any other provision of the Offer, ILN will not
be required to accept for payment or pay for any Shares tendered, and may
terminate or amend the Offer (by oral or written notice to the Depositary and
timely public announcement) or may postpone (subject to the requirements of the
Exchange Act for prompt payment for or return of Shares) the acceptance for
payment of, or payment for, Shares tendered, if at any time on or after [DAY
BEFORE STATEMENT DATE], and at or before the Expiration Date, any of the
following shall have occurred (which shall not have been waived by ILN):
 
          (i) there shall have been threatened, instituted or pending any action
     or proceeding by any government or governmental, regulatory or
     administrative agency, authority or tribunal or any other person, domestic
     or foreign, or before any court, authority, agency or tribunal that (a)
     challenges the acquisition of Shares pursuant to the Offer or otherwise in
     any manner relates to or affects the Offer or (b) in the reasonable
     judgment of ILN, would or might materially and adversely affect the
     business, condition (financial or otherwise), income, operations or
     prospects of ILN and its subsidiaries taken as a whole, or otherwise
     materially impair in any way the contemplated future conduct of the
     business of ILN or any of its subsidiaries or materially impair the Offer's
     contemplated benefits to ILN;
 
          (ii) there shall have been any action threatened, pending or taken, or
     approval withheld, or any statute, rule, regulation, judgment, order or
     injunction threatened, proposed, sought, promulgated, enacted, entered,
     amended, enforced or deemed to be applicable to the Offer or ILN or any of
     its subsidiaries, by any legislative body, court, authority, agency or
     tribunal that, in ILN's reasonable judgment, would or might directly or
     indirectly (a) make the acceptance for payment of, or payment for, some or
     all of the Shares illegal or otherwise restrict or prohibit consummation of
     the Offer; (b) delay or restrict the ability of ILN, or render ILN unable,
     to accept for payment or pay for some or all of the Shares; (c) materially
     impair the contemplated benefits of the Offer to ILN or IPC (including
     materially increasing the effective interest cost of certain types of
     unsecured indebtedness); or (d) materially affect the business, condition
     (financial or otherwise), income, operations or prospects of ILN and its
     subsidiaries taken as a whole, or otherwise materially impair in any way
     the contemplated future conduct of the business of ILN or any of its
     subsidiaries;
 
          (iii) there shall have occurred (a) any significant decrease in the
     market price of the Shares, (b) any change in the general political,
     market, economic or financial conditions in the United States or abroad
     that, in the reasonable judgment of ILN, would or might have a material
     adverse effect on ILN's business, operations, prospects or ability to
     obtain financing generally or the trading in the Shares or other equity
     securities of ILN; (c) the declaration of a banking moratorium or any
     suspension of payments in respect of banks in the United States or any
     limitation on, or any event that, in ILN's reasonable judgment, would or
     might affect the extension of credit by lending institutions in the United
     States; (d) the commencement of war, armed hostilities or other
     international or national calamity directly or indirectly involving the
     United States; (e) any general suspension of trading in, or limitation on
     prices for, securities on any national securities exchange or in the
     over-the-counter market; (f) in the case of any of the foregoing existing
     at the time of the commencement of the Offer, in ILN's reasonable judgment,
     a material acceleration or worsening thereof, (g) any decline in either the
     Dow Jones Industrial Average or the Standard and Poor's Composite 500 Stock
     Index by an amount in excess of 15% measured from the close of business on
     [DAY BEFORE STATEMENT DATE]; or (h) a decline in the ratings accorded any
     of ILN's securities or the Shares by Moody's, S&P or D&P, or that S&P,
     Moody's or D&P has announced that it has placed any such rating under
     surveillance or review with negative implications;
 
          (iv) any tender or exchange offer with respect to some or all of the
     Shares (other than the Offer) or other equity securities of ILN, or a
     merger, acquisition or other business combination proposal for ILN, shall
     have been proposed, announced or made by any person or entity; or
 
          (v) there shall have occurred any event or events that have resulted,
     or, in ILN's reasonable judgment, may result, in an actual or threatened
     change in the business, condition (financial or otherwise), income,
     operations, stock ownership or prospects of ILN and its subsidiaries;
 
and in the sole judgment of ILN, such event or events make it undesirable or
inadvisable to proceed with the Offer or with such acceptance for payment or
payment.
                                       17
<PAGE>   18
 
     The foregoing conditions (including the condition that the Proposed
Amendment be adopted at the Special Meeting) are for the sole benefit of ILN and
may be asserted by ILN regardless of the circumstances (including any action or
inaction by ILN) giving rise to any such condition, and any such condition may
be waived by ILN, in whole or in part, at any time and from time to time in its
sole discretion. A decision by ILN to terminate or otherwise amend any Offer,
following the occurrence of any of the foregoing, with respect to one Series of
Preferred will not create an obligation on behalf of ILN to terminate or
otherwise amend in a similar manner the Offer with respect to any other Series
of Preferred. The failure by ILN at any time to exercise any of the foregoing
rights shall not be deemed a waiver of any such right and each such right shall
be deemed an ongoing right which may be asserted at any time and from time to
time. Any determination by ILN concerning the events described above will be
final and binding on all parties.
 
EXTENSION OF TENDER PERIOD; TERMINATION; AMENDMENTS
 
     ILN expressly reserves the right, in its sole discretion, and at any time
and/or from time to time prior to the Expiration Date, to extend the period of
time during which the Offer for any Series of Preferred is open by giving oral
or written notice of such extension to the Depositary, without extending the
period of time during which the Offer for any other Series of Preferred is open.
There can be no assurance, however, that ILN will exercise its right to extend
the Offer for any Series of Preferred. During any such extension, all Shares of
the subject Series of Preferred previously tendered will remain subject to the
Offer, except to the extent that such Shares may be withdrawn as set forth in
"Terms of the Offer -- Withdrawal Rights."
 
     ILN also expressly reserves the right, in its sole discretion, to, among
other things, terminate the Offer and not accept for payment or pay for any
Shares tendered, subject to Rule 13e-4(f)(5) under the Exchange Act, which
requires ILN either to pay the consideration offered or to return the Shares
tendered promptly after the termination or withdrawal of the Offer upon the
occurrence of any of the conditions specified in "Terms of the Offer -- Certain
Conditions of the Offer" by giving oral or written notice of such termination to
the Depositary, and making a public announcement thereof.
 
   
     Subject to compliance with applicable law, ILN further reserves the right,
in its sole discretion, to amend the Offer in any respect. Amendments to the
Offer may be made at any time and/or from time to time effected by public
announcement thereof, such announcement, in the case of an extension, to be
issued no later than 9:00 a.m., New York City time, on the next business day
after the previously scheduled Expiration Date. Any public announcement made
pursuant to the Offer will be disseminated promptly to Preferred Shareholders
affected thereby in a manner reasonably designed to inform such Preferred
Shareholders of such change. Without limiting the manner in which ILN may choose
to make a public announcement, except as required by applicable law, ILN shall
have no obligation to publish, advertise or otherwise communicate any such
public announcement other than by issuing a press release to the Dow Jones News
Service.
    
 
   
     If ILN materially changes the terms of the Offer or the information
concerning the Offer, or if it waives a material condition of the Offer, ILN
will extend the Offer to the extent required by Rules 13e-4(d)(2) and
13e-4(e)(2) under the Exchange Act. Those rules require that the minimum period
during which the Offer must remain open following material changes in the terms
of the Offer or information concerning the Offer (other than a change in price,
a change in percentage of securities sought or a change in the dealer's
solicitation fee) will depend on the facts and circumstances, including the
relative materiality of such terms or information. The SEC has stated that, in
its view, an offer should remain open for a minimum of five business days from
the date that a notice of such a material change is first published, sent or
given. If the Offer is scheduled to expire at any time earlier than the
expiration of a period ending on the tenth business day from, and including, the
date that ILN publishes, sends or gives to Preferred Shareholders a notice that
it will (i) increase or decrease the price it will pay for Shares, (ii) decrease
the percentage of Shares it seeks or (iii) increase or decrease the Soliciting
Dealers' Fees, the Offer will be extended until the expiration of such period of
ten business days. THE OFFER FOR EACH SERIES OF PREFERRED IS INDEPENDENT OF THE
OFFER FOR ANY OTHER SERIES OF PREFERRED. IF ILN EXTENDS OR AMENDS ANY OFFER WITH
RESPECT TO ONE SERIES OF PREFERRED FOR ANY REASON, ILN WILL HAVE NO OBLIGATION
TO EXTEND THE OFFER FOR ANY OTHER SERIES OF PREFERRED.
    
 
                                       18
<PAGE>   19
 
                   PROPOSED AMENDMENT AND PROXY SOLICITATION
 
INTRODUCTION
 
     This Offer to Purchase and Proxy Statement is first being mailed on or
about [STATEMENT DATE] to the Preferred Shareholders of IPC in connection with
the solicitation of proxies by the Board of Directors of IPC (the "Board") for
use at the Special Meeting. At the Special Meeting, the shareholders of record
of IPC will vote upon the Proposed Amendment to the Articles.
 
   
     While Preferred Shareholders who wish to tender their Shares pursuant to
the Offer are not required to vote in favor of the Proposed Amendment, the Offer
is conditioned upon the Proposed Amendment being adopted at the Special Meeting.
In addition, Preferred Shareholders have the right to vote for the Proposed
Amendment regardless of whether they tender their Shares. If the Proposed
Amendment is adopted at the Special Meeting, IPC will make a special cash
payment in the amount of $     per Share (the "Special Cash Payment") to each
Preferred Shareholder of record as of the Record Date who did not tender his or
her Shares in the Offer. Those Preferred Shareholders who validly tender their
Shares will be entitled only to the applicable purchase price per Share listed
on the front cover of this Offer to Purchase and Proxy Statement plus accrued
and unpaid dividends for the Shares tendered. See "-- Special Cash Payments."
    
 
VOTING SECURITIES, RIGHTS AND PROCEDURES
 
   
     Only holders of record of IPC's voting securities at the close of business
on the Record Date will be entitled to vote in person or by proxy at the Special
Meeting. The outstanding voting securities of IPC are divided into two classes:
common stock and cumulative preferred stock (par value $50 per Share). The class
of cumulative preferred stock has been issued in six Series of Preferred with
the record holders of all Shares of the cumulative preferred stock voting
together as one class. The shares outstanding as of the Record Date, and the
vote to which each share is entitled in consideration of the Proposed Amendment,
are as follows:
    
 
<TABLE>
<CAPTION>
                                                              SHARES       VOTES PER
                         CLASS                              OUTSTANDING      SHARE
                         -----                              -----------    ---------
<S>                                                         <C>            <C>
Common Stock (No Par Value).............................     66,215,292     1 vote
Cumulative Preferred Stock (Par Value $50 Per Share)....      1,139,110     1 vote
</TABLE>
 
   
     The affirmative votes of the holders of at least two-thirds of the
outstanding shares of IPC's (i) common stock and all series of Cumulative
Preferred Stock, voting together as one class, and (ii) Cumulative Preferred
Stock, all series voting together as one class, are required to approve the
Proposed Amendment to be presented at the Special Meeting. Abstentions and
broker non-votes will have the effect of votes against the Proposed Amendment.
ILN, WHICH OWNS ALL OF THE OUTSTANDING SHARES OF COMMON STOCK OF IPC, HAS
ADVISED IPC THAT IT INTENDS TO VOTE ALL OF THE OUTSTANDING SHARES OF COMMON
STOCK OF IPC IN FAVOR OF THE PROPOSED AMENDMENT.
    
 
     Votes at the Special Meeting will be tabulated preliminarily by the
Depositary. Inspectors of Election, duly appointed by the presiding officer of
the Special Meeting, will definitively count and tabulate the votes and
determine and announce the results at the Special Meeting. IPC has no
established procedure for confidential voting.
 
DISSENTERS' RIGHTS
 
     Section 11.65 of the Illinois Business Corporation Act of 1983, as amended
(the "Business Corporation Act"), provides that a shareholder of an Illinois
corporation is entitled to dissent from, and obtain payment for his or her
shares in the event of, among other things, an amendment of the corporation's
articles of incorporation that materially and adversely affects rights in
respect of a dissenter's shares because it alters or abolishes a preferential
right of such shares. As described herein, the Proposed Amendment would amend
the Articles to remove a covenant which provides that the issuance or assumption
of unsecured indebtedness for certain purposes above a certain threshold
requires the consent of the holders of at least a majority of the total number
of Shares then outstanding. To the extent that the Proposed Amendment is deemed
to alter or abolish a "preferential right" of the Shares within the meaning of
Section 11.65 of the Business Corporation Act, the
 
                                       19
<PAGE>   20
 
   
Preferred Shareholders would have the right to dissent from the Proposed
Amendment and obtain payment of the fair value of their Shares. Pursuant to
Section 11.70 of the Business Corporation Act, a Preferred Shareholder who
elects to dissent from the Proposed Amendment (a "Dissenter"), must deliver to
IPC before                     , 1998 a written demand for payment of his or her
Shares if the Proposed Amendment is adopted at the Special Meeting. If a written
demand is not received by that date, the Dissenter will lose the right to
dissent and thereby obtain payment for his or her Shares. A Dissenter also will
lose the right to dissent if the Dissenter votes in favor of the Proposed
Amendment.
    
 
     If the Proposed Amendment gives rise to dissenters' rights under Section
11.65 of the Business Corporation Act and is adopted at the Special Meeting, IPC
will advise any Dissenter meeting the foregoing eligibility requirements of its
opinion as to the estimated fair value of the Shares owned by the Dissenter,
together with supporting information and a commitment to pay for such Shares at
the estimated fair value. A Dissenter who does not agree with IPC's estimated
fair value of the Shares must notify IPC, within 30 days thereafter, of the
Dissenter's estimate of fair value. If, within 60 days thereafter, IPC and the
Dissenter have not agreed upon the fair value of the Shares and the interest
due, IPC must either pay the Dissenter the difference in value demanded by the
Dissenter or file a petition in circuit court requesting the court to determine
the fair value of the Shares and the interest due.
 
     The foregoing summary does not purport to be a complete statement of the
provisions of Sections 11.65 and 11.70 of the Business Corporation Act and is
qualified in its entirety by reference to the relevant portions of such
Sections, copies of which are attached hereto as Attachment A.
 
     A Dissenter who receives payment for his or her Shares upon exercise of the
right of dissent will, subject to the provisions of the Section 302(b) of the
Internal Revenue Code, recognize gain or loss for Federal income tax purposes,
measured by the difference between the cost basis for his or her Shares and the
amount of payment received.
 
     PREFERRED SHAREHOLDERS WHO WISH TO RESERVE THE RIGHT TO EXERCISE
DISSENTERS' RIGHTS SHOULD REVIEW CAREFULLY THE FOREGOING DISCUSSION AND THE
PROVISIONS OF THE BUSINESS CORPORATION ACT SET FORTH IN ATTACHMENT A. THE
FAILURE TO COMPLY STRICTLY WITH THE DESCRIBED PROCEDURES WILL RESULT IN THE LOSS
OF ANY SUCH DISSENTERS' RIGHTS. ANY PREFERRED SHAREHOLDER WHO CONTEMPLATES THE
ASSERTION OF DISSENTERS' RIGHTS IS URGED TO CONSULT HIS OR HER OWN COUNSEL.
 
PROXIES
 
     THE ENCLOSED PROXY (WHICH IS INCLUDED AS PART OF THE LETTER OF TRANSMITTAL
AND PROXY AND WHICH IS ALSO ENCLOSED SEPARATELY HEREWITH) IS SOLICITED BY IPC'S
BOARD, WHICH RECOMMENDS VOTING IN FAVOR OF THE PROPOSED AMENDMENT. ALL SHARES OF
IPC'S COMMON STOCK WILL BE VOTED IN FAVOR OF THE PROPOSED AMENDMENT. Shares of
IPC's cumulative preferred stock represented by properly executed proxies
received at or prior to the Special Meeting will be voted in accordance with the
instructions thereon. If no instructions are indicated, duly executed proxies
will be voted in accordance with the recommendation of the Board. It is not
anticipated that any other matters will be brought before the Special Meeting.
However, the enclosed proxy gives discretionary authority to the proxy holders
named therein should any other matters be presented at the Special Meeting, and
it is the intention of the proxy holders to act on any other matters in
accordance with their best judgment.
 
     Execution of a proxy will not prevent a shareholder from attending the
Special Meeting and voting in person. Any shareholder giving a proxy may revoke
it at any time before it is voted by delivering to the Secretary of IPC written
notice of revocation bearing a later date than the proxy, by delivering a duly
executed proxy bearing a later date, or by voting in person by ballot at the
Special Meeting. Withdrawal of Shares tendered pursuant to the Offer will not
revoke a properly executed proxy.
 
     IPC will bear the cost of the solicitation of proxies by the Board. IPC has
engaged MacKenzie Partners, Inc. to act as Information Agent in connection with
the solicitation of proxies for a fee of $       , plus reimbursement of
reasonable out-of-pocket expenses. Proxies will be solicited by mail, telephone
or other
 
                                       20
<PAGE>   21
 
   
electronic means. In addition, officers and employees of IPC may also solicit
proxies personally or by telephone; such persons will receive no additional
compensation for these services. The Information Agent has not been retained to
make, and will not make, solicitations or recommendations in connection with the
Proposed Amendment.
    
 
     IPC has requested that brokerage firms and other custodians, nominees and
fiduciaries forward solicitation materials to the beneficial owners of shares of
IPC's cumulative preferred stock held of record by such persons and will
reimburse such brokers and other fiduciaries for their reasonable out-of-pocket
expenses incurred in connection therewith.
 
SPECIAL CASH PAYMENTS
 
   
     Subject to the terms and conditions set forth in this Offer to Purchase and
Proxy Statement, if (but only if) the Proposed Amendment is adopted at the
Special Meeting, IPC will make a Special Cash Payment in the amount of $  for
each Share to each Preferred Shareholder of record as of the Record Date who did
not tender his or her Shares in the Offer. SPECIAL CASH PAYMENTS WILL BE MADE TO
PREFERRED SHAREHOLDERS AS OF THE RECORD DATE (IF SUCH SHARES HAVE NOT BEEN
TENDERED PURSUANT TO THE OFFER). THOSE PREFERRED SHAREHOLDERS WHO VALIDLY TENDER
THEIR SHARES WILL BE ENTITLED ONLY TO THE APPLICABLE PURCHASE PRICE PER SHARE
LISTED ON THE FRONT COVER OF THIS OFFER TO PURCHASE AND PROXY STATEMENT PLUS
ACCRUED AND UNPAID DIVIDENDS FOR THE SHARES TENDERED. IPC has been advised that
there is uncertainty under state law, due to the lack of controlling precedent,
as to the permissibility of making Special Cash Payments. While IPC cannot
predict how a court would rule on the issue, IPC believes that ILN's Offer is
fair to Preferred Shareholders, that the Proposed Amendment is in the best
interests of IPC and its shareholders, and, accordingly, has decided to make the
Special Cash Payments. If the Proposed Amendment is adopted at the Special
Meeting, Special Cash Payments will be paid out of IPC's general funds, promptly
after the Proposed Amendment shall have become effective. However, no accrued
interest will be paid on the Special Cash Payments regardless of any delay in
making such payments.
    
 
     Only Preferred Shareholders on the Record Date (or their legal
representatives or attorneys-in-fact) are entitled to vote at the Special
Meeting and, if the Proposed Amendment is adopted at the Special Meeting, to
receive Special Cash Payments from IPC. Any beneficial holder of Shares who is
not the registered holder of such Shares as of the Record Date (as would be the
case for any beneficial holder whose Shares are registered in the name of such
holder's broker, dealer, commercial bank, trust company or other nominee) must
arrange with the record Preferred Shareholder to execute and deliver a proxy
form on such beneficial owner's behalf. If a beneficial holder of Shares intends
to attend the Special Meeting and vote in person, such beneficial holder must
obtain a legal proxy form from his or her broker, dealer, commercial bank, trust
company or other nominee.
 
   
     TO AVOID FEDERAL INCOME TAX BACKUP WITHHOLDING EQUAL TO 31% OF THE SPECIAL
CASH PAYMENT, EACH PREFERRED SHAREHOLDER MUST NOTIFY THE DEPOSITARY OF SUCH
PREFERRED SHAREHOLDER'S CORRECT TAXPAYER IDENTIFICATION NUMBER AND PROVIDE
CERTAIN OTHER INFORMATION BY PROPERLY COMPLETING AND EXECUTING THE SUBSTITUTE
FORM W-9 INCLUDED IN THE LETTER OF TRANSMITTAL AND PROXY OR THE SUBSTITUTE FORM
W-9 INCLUDED IN THE SEPARATE PROXY. FOREIGN PREFERRED SHAREHOLDERS MUST SUBMIT A
PROPERLY COMPLETED FORM W-8 IN ORDER TO AVOID THE APPLICABLE BACKUP WITHHOLDING;
PROVIDED, HOWEVER, THAT BACKUP WITHHOLDING WILL NOT APPLY TO FOREIGN PREFERRED
SHAREHOLDERS SUBJECT TO 30% (OR LOWER TREATY RATE) WITHHOLDING ON SPECIAL CASH
PAYMENTS. SEE "CERTAIN FEDERAL INCOME TAX CONSEQUENCES."
    
 
SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT
 
     IPC Securities. As noted above, ILN owns all the outstanding common stock
of IPC. ILN, IPC and their subsidiaries and affiliates do not beneficially own
any Shares as of January 31, 1998. None of the pension, profit sharing or other
retirement plans of such entities beneficially own any Shares as of January 31,
1998. In
 
                                       21
<PAGE>   22
 
   
addition, IPC's and ILN's directors and executive officers do not beneficially
own any Shares as of January 31, 1998. There is no person or group known by
management of IPC to be the beneficial owner of more than 5% of the Shares as of
January 31, 1998, except for the State of Michigan Retirement Systems, 430 W.
Allegan, Lansing, MI 48909, which beneficially owns 150,000, or 13.17%, of the
Shares.
    
 
   
     ILN Securities. The beneficial ownership of ILN's common stock held by each
director of ILN and IPC, the five most highly compensated executive officers of
ILN and IPC in 1997, all directors and executive officers as a group of ILN and
IPC, and persons or groups owning more than 5%, as of January 31, 1998 (unless
otherwise noted), is set forth in the following table.
    
 
   
<TABLE>
<CAPTION>
                                                              AMOUNT AND NATURE OF
                 NAME OF BENEFICIAL OWNER                    BENEFICIAL OWNERSHIP(1)   PERCENT OF CLASS
                 ------------------------                    -----------------------   ----------------
<S>                                                          <C>                       <C>
Larry F. Altenbaumer.......................................            13,092(2)              *
David W. Butts.............................................             8,817                 *
John G. Cook...............................................            11,894(2)              *
Larry D. Haab..............................................            68,250(2)              *
Paul L. Lang...............................................            21,216(2)              *
Robert A. Schultz..........................................             8,551(2)              *
J. Joe Adorjan.............................................                 0                 *
C. Steven McMillan.........................................             1,300                 *
Robert M. Powers...........................................             8,550                 *
Sheli Z. Rosenberg.........................................                 0                 *
Walter D. Scott............................................             5,150                 *
Joe J. Stewart.............................................                 0                 *
Ronald L. Thompson.........................................             3,677                 *
Walter M. Vannoy...........................................             5,010                 *
Marilou von Ferstel........................................             4,420                 *
John D. Zeglis.............................................             2,626                 *
All directors and executive officers as a group............           187,021                 *
Hotchkis & Wiley...........................................         6,321,233              8.8%
  800 W. 6th Street, 5th Floor
  Los Angeles, CA 90017
Merrill Lynch Asset Management, L.P. ......................         6,321,253(3)           8.8%
  800 Scudders Mill Road
  Plainsboro, NJ 08536
State of Michigan Retirement Systems.......................         3,714,300             5.18%
  430 W. Allegan
  Lansing, MI 48909
</TABLE>
    
 
- -------------------------
* Less than 1%.
 
   
(1) Each person named in the table has sole voting and investment power with
    respect to shares of ILN's common stock listed as owned by each person,
    unless otherwise noted.
    
   
(2) Includes the following shares of common stock issuable pursuant to stock
    options exercisable within the next 60 days: Mr. Haab, 56,900; Mr. Lang,
    17,800; Mr. Altenbaumer, 17,800; Mr. Butts, 7,900; Mr. Cook, 9,900; and Mr.
    Schultz, 6,750.
    
   
(3) Merrill Lynch Asset Management, L.P. has shared voting and investment power
    with respect to these shares according to its Schedule 13G filed February 2,
    1998.
    
 
                                       22
<PAGE>   23
 
BUSINESS TO COME BEFORE THE SPECIAL MEETING
 
     The following Proposed Amendment to the Articles is the only item of
business expected to be presented at the Special Meeting:
 
        To remove in its entirety ARTICLE V, Section 1, Clause (f)(1), limiting
        IPC's ability to issue or assume unsecured indebtedness (the "Debt
        Limitation Provision").
 
     THE FOLLOWING STATEMENTS, UNLESS THE CONTEXT OTHERWISE REQUIRES, ARE
SUMMARIES OF THE SUBSTANCE OR GENERAL EFFECT OF PROVISIONS OF THE ARTICLES, AND
ARE QUALIFIED IN THEIR ENTIRETY BY THE ARTICLES, INCLUDING ARTICLE V SECTION 1,
CLAUSE (F)(1) (AS DESCRIBED BELOW).
 
EXPLANATION OF THE PROPOSED AMENDMENT
 
     The Debt Limitation Provision in the Articles currently prohibits, without
the consent of the holders of a majority of the outstanding Shares, the issuance
or assumption by IPC of any unsecured notes, debentures or other securities
representing unsecured indebtedness (other than for the purpose of refunding
outstanding unsecured indebtedness or for the redemption or retirement of all
outstanding Shares) if, immediately after such issuance or assumption, the total
outstanding principal amount of all securities representing unsecured
indebtedness (including unsecured securities then to be issued or assumed) would
exceed 20% of the aggregate of (i) the total principal amount of all outstanding
secured indebtedness issued or assumed by IPC at the time of such issuance or
assumption and (ii) the capital and surplus of IPC, as then stated on IPC's
books of account. The Proposed Amendment, if adopted, would eliminate in its
entirety the Debt Limitation Provision, as set forth below, from the Articles:
 
          (f)(1) The Corporation shall not, without the consent (given by vote
     at a meeting called for that purpose) of the holders of at least a majority
     of the total number of shares of Preferred Stock then outstanding, issue
     any unsecured notes, debentures or other securities representing unsecured
     indebtedness, or assume any such unsecured indebtedness, for purposes other
     than (A) the refunding of outstanding unsecured indebtedness theretofore
     issued or assumed by the Corporation, (B) the reacquisition, redemption or
     other retirement of any indebtedness which reacquisition, redemption or
     other retirement has been approved by any regulatory authority having
     jurisdiction in the premises, or (C) the reacquisition, redemption or other
     retirement of all outstanding shares of Preferred Stock or of any other
     class of stock ranking prior to, or on a parity with, Preferred Stock as to
     dividends or other distributions, if immediately after such issue or
     assumption the total principal amount of all unsecured notes, debentures or
     other securities representing unsecured indebtedness issued or assumed by
     the Corporation including unsecured indebtedness then to be issued or
     assumed would exceed twenty percent (20%) of the aggregate of (a) the total
     principal amount of all bonds or other securities representing secured
     indebtedness issued or assumed by the Corporation and then to be
     outstanding, and (b) the capital and surplus of the Corporation as then to
     be stated on the books of account of the Corporation.
 
REASONS FOR THE PROPOSED AMENDMENT
 
     General. IPC believes that legislative, regulatory, technological and
market developments will lead to a more competitive environment in the electric
utility industry. As competition intensifies, flexibility and cost reduction
will be even more crucial to success. Because the electric utility industry is
extremely capital intensive, control and minimization of financing costs are of
particular importance. In response to the competitive forces and regulatory
changes faced by IPC, IPC has from time to time considered, and expects to
continue to consider, various strategies designed to enhance its competitive
position and to increase its ability to adapt to and anticipate changes in its
utility business.
 
     IPC believes that adoption of the Proposed Amendment is critical to
financial flexibility and capital cost reduction. If the Proposed Amendment is
adopted, the Debt Limitation Provision will be eliminated. Historically, IPC's
debt financing generally has been accomplished through the issuance of long-term
mortgage bonds, unsecured indebtedness and pollution control bonds. IPC has
mortgage bonds outstanding under two mortgages: (i) the 1943 Mortgage and Deed
of Trust, between IPC and Harris Trust and Savings
 
                                       23
<PAGE>   24
 
Bank (the "1943 Mortgage"), and (ii) the 1992 General Mortgage and Deed of
Trust, between IPC and Harris Trust and Savings Bank (the "1992 Mortgage").
Mortgage bonds issued under the 1943 Mortgage represent secured indebtedness
placing a first priority lien on substantially all of IPC's assets. While
subject to the prior lien of the 1943 Mortgage, IPC's utility assets are also
subject to the junior lien of the 1992 Mortgage. IPC's mortgage bonds issued
under both the 1943 Mortgage and the 1992 Mortgage contain certain restrictive
covenants with respect to, among other things, the disposition of assets and the
ability to issue additional mortgage bonds. Unsecured indebtedness generally has
fewer restrictions than mortgage bonds. Short-term indebtedness, a lower cost
form of debt available to IPC, represents one type of unsecured indebtedness.
Pollution control bond financing, a more favorable type of financing due to its
tax-exempt status, is available only for very limited purposes.
 
   
     The Proposed Amendment will not only allow IPC to issue a greater amount of
unsecured indebtedness, but also will allow IPC to issue a greater amount of
total indebtedness. IPC, however, presently has no intention of issuing a
greater amount of unsecured debt or total debt than it would have issued absent
the adoption of the Proposed Amendment. As described herein, IPC's present
plans, which are subject to change based upon market conditions and other
factors, contemplate that (i) IPC may issue additional unsecured debt in the
event it determines to purchase the Shares from ILN and (ii) IPC may issue debt
in connection with a securitization financing authorized under PA 90-561. It is
IPC's intention to retain flexibility in the mix of its outstanding debt and
therefore have the option to use more short-term and other unsecured debt and
fewer mortgage bonds. In addition, as a regulated utility, the issuance of any
securities by IPC would continue to be subject to the prior approval of the ICC
(with respect to securities maturing in more than one year) or the Federal
Energy Regulatory Commission (with respect to securities maturing in one year or
less).
    
 
     Inasmuch as the Debt Limitation Provision contained in the Articles limits
IPC's flexibility in planning and financing its business activities, IPC
believes it ultimately will be at a competitive disadvantage if the Debt
Limitation Provision is not eliminated. The industry's new competitors (for
example, power marketers, exempt wholesale generators, independent power
producers and cogeneration facilities) generally are not subject to the type of
financing restrictions the Articles impose on IPC. Recently, several other
utilities with the same or similar charter restrictions have successfully
eliminated such provisions by soliciting their shareholders for the same or
similar amendments. In addition, some potential utility competitors have no
comparable provision restricting the issuance of unsecured debt. Given the onset
of competition in the utility industry, IPC must continue to explore new ways of
reducing costs and enhancing flexibility. IPC believes that the adoption of the
Proposed Amendment will be in the best long-term competitive interests of its
shareholders.
 
   
     Since holders of any debt, including unsecured indebtedness, would rank
ahead of Preferred Shareholders in a bankruptcy or other liquidation of IPC, the
issuance of a greater amount of unsecured indebtedness may pose additional risk
to Preferred Shareholders under certain circumstances, including circumstances
relating to IPC's default in the repayment of such indebtedness. Reference is
also made to "Special Factors -- Potential Effects of the Proposed Amendment on
Non-Tendering Preferred Shareholders" for a discussion of certain other
consequences resulting from the elimination of the Debt Limitation Provision.
    
 
     Financial Flexibility. If the Proposed Amendment is adopted, IPC will have
increased flexibility (i) to choose among different types of debt financing and
(ii) to finance projects using the most cost effective means. IPC believes that
various types of unsecured debt alternatives may increase in importance as a
financing option. The availability and flexibility of unsecured debt is
necessary to take full advantage of changing conditions in securities markets.
 
     In addition, although IPC's earnings currently are sufficient to meet the
earnings coverage tests that must be satisfied before issuing additional
mortgage bonds and preferred stock, there is no guarantee that this will be true
in the future. Other utilities have been unable to issue mortgage bonds during
certain periods because of restrictive covenants in their mortgages. IPC's
inability to issue mortgage bonds or preferred stock in the future, combined
with the inability to issue additional unsecured debt, would limit IPC's
financing options to more costly options, including additional common equity.
Moreover, continued reliance on the issuance of mortgage bonds could limit IPC's
ability in the future to strategically redeploy its assets.
 
                                       24
<PAGE>   25
 
     Under the Debt Limitation Provision, IPC's use of unsecured short-term
indebtedness is presently restricted. However, IPC believes that the prudent use
of such indebtedness in excess of this provision is vital to effective financial
management of its business. Not only is unsecured short-term indebtedness
generally one of the least expensive forms of capital, it also provides
flexibility in meeting seasonal and business cycle fluctuations in cash
requirements, acts as a bridge between issues of permanent capital and can be
used when unfavorable conditions prevail in the market for long-term capital.
However, because of the Debt Limitation Provision, as of December 31, 1997, the
maximum amount of unsecured indebtedness that IPC would be authorized under its
Articles to issue or assume would be approximately $612.7 million. As of
December 31, 1997, IPC had approximately $444.8 million of unsecured
indebtedness outstanding, thus leaving an additional $167.9 million of capacity
available.
 
   
     Lower Costs. As previously mentioned, IPC's short-term debt issuances
generally represent one of its lowest-cost forms of financing. By increasing its
use of short-term debt, IPC may be able to lower its cost structure further,
thereby making its products more competitive and reducing its business risks.
However, with the Debt Limitation Provision in place, the availability and
corresponding benefits of short-term debt diminish. In addition, although
short-term debt may expose the borrower to more volatility in interest rates, it
should be noted that the cost of short-term debt seldom exceeds the cost of
other forms of capital available at the same time.
    
 
     FOR ALL THE ABOVE REASONS, IPC'S BOARD BELIEVES THE BEST LONG-TERM
INTERESTS OF SHAREHOLDERS ARE SERVED BY, AND ENCOURAGES SHAREHOLDERS TO VOTE IN
FAVOR OF, THE ADOPTION OF THE PROPOSED AMENDMENT.
 
RELATIONSHIP WITH INDEPENDENT ACCOUNTANTS
 
     ILN's Board of Directors has selected Price Waterhouse LLP as independent
accountants for ILN and its subsidiaries, including IPC, for the year 1998. A
representative of Price Waterhouse LLP is expected to be present at the Special
Meeting with the opportunity to make a statement and to respond to appropriate
questions from shareholders.
 
                        PRICE RANGE OF SHARES; DIVIDENDS
 
     Shares of the 4.08% Series, 4.20% Series, 4.26% Series, 4.42% Series and
4.70% Series are listed and traded on the NYSE under the symbols "IPCprA,"
"IPCprB," "IPCprC," "IPCprD" and "IPCprE," respectively. The last reported sales
price on the NYSE, as of the close of business on [DAY BEFORE STATEMENT DATE],
for each of these Series of Preferred is shown on the front cover of this Offer
to Purchase and Proxy Statement.
 
     Shares of the 7.75% Series trade in the over-the-counter market under the
symbol "ILLNP". The last reported sales price in the over-the-counter market, as
of the close of business on [DAY BEFORE STATEMENT DATE], for the 7.75% Series of
Preferred is shown on the front cover of this Offer to Purchase and Proxy
Statement. However, Preferred Shareholders should be aware that there is no
established trading market for these Shares and that the Shares of the Series of
Preferred only trade sporadically and on a limited basis and, therefore, the
last reported sales price may not necessarily reflect the current market value
of such Shares.
 
   
     On the Record Date, there were issued and outstanding 283,290 shares of the
4.08% Series of Preferred held by shareholders of record; 167,720 shares of the
4.20% Series of Preferred held by shareholders of record; 136,000 shares of the
4.26% Series of Preferred held by shareholders of record; 134,400 shares of the
4.42% Series of Preferred held by shareholders of record; 176,000 shares of the
4.70% Series of Preferred held by shareholders of record; and 241,700 shares of
the 7.75% Series of Preferred held by shareholders of record.
    
 
     PREFERRED SHAREHOLDERS ARE URGED TO OBTAIN CURRENT MARKET QUOTATIONS, IF
AVAILABLE, FOR THE SHARES.
 
                                       25
<PAGE>   26
 
     The following table sets forth the high and low sales prices of each Series
of Preferred on the NYSE or in the over-the-counter market as reported by the
National Quotation Bureau, Inc., as applicable, and the cash dividends paid
thereon for the fiscal quarters indicated.
 
            DIVIDENDS AND PRICE RANGES OF CUMULATIVE PREFERRED STOCK
                          BY QUARTERS (1997 AND 1996)
 
<TABLE>
<CAPTION>
                                         1997 -- QUARTERS                                    1996 -- QUARTERS
                        --------------------------------------------------  --------------------------------------------------
                            1ST          2ND          3RD          4TH          1ST          2ND          3RD          4TH
                            ---          ---          ---          ---          ---          ---          ---          ---
<S>                     <C>          <C>          <C>          <C>          <C>          <C>          <C>          <C>
CUMULATIVE PREFERRED
  STOCK ($50 Par
  Value)
4.08% Series
  Dividends Paid Per
    Share.............     $0.51        $0.51        $0.51        $0.51        $0.51        $0.51        $0.51        $0.51
  Market Price -- $
    Per Share (NYSE)
    (High/Low)........  30.00/25.50  29.00/25.50  28.50/25.38  31.00/27.25  31.50/27.00  31.00/27.88  33.53/29.13  36.00/30.44
4.20% Series
  Dividends Paid Per
    Share.............   $0.52 1/2    $0.52 1/2    $0.52 1/2    $0.52 1/2    $0.52 1/2    $0.52 1/2    $0.52 1/2    $0.52 1/2
  Market Price -- $
    Per Share (NYSE)
    (High/Low)........  30.00/27.00  29.00/27.38  30.00/26.88  32.00/28.50  32.00/28.00  32.00/27.00  34.52/30.50  37.00/32.00
4.26% Series
  Dividends Paid Per
    Share.............   $0.53 1/4    $0.53 1/4    $0.53 1/4    $0.53 1/4    $0.53 1/4    $0.53 1/4    $0.53 1/4    $0.53 1/4
  Market Price -- $
    Per Share (NYSE)
    (High/Low)........  31.00/27.38  29.50/27.38  32.00/26.75  32.13/28.00  33.00/28.25  32.50/29.88  34.50/30.94  37.50/32.50
4.42% Series
  Dividends Paid Per
    Share.............   $0.55 1/4    $0.55 1/4    $0.55 1/4    $0.55 1/4    $0.55 1/4    $0.55 1/4    $0.55 1/4    $0.55 1/4
  Market Price -- $
    Per Share (NYSE)
    (High/Low)........  32.00/29.00  31.50/27.75  32.00/27.13  34.00/29.00  33.50/29.50  34.00/29.50  35.50/32.00  39.00/33.25
4.70% Series
  Dividends Paid Per
    Share.............   $0.58 3/4    $0.58 3/4    $0.58 3/4    $0.58 3/4    $0.58 3/4    $0.58 3/4    $0.58 3/4    $0.58 3/4
  Market Price -- $
    Per Share (NYSE)
    (High/Low)........  34.00/30.38  33.00/29.38  33.00/29.63  36.00/31.13  35.50/31.00  36.00/33.00  38.63/33.50  41.00/36.00
7.75% Series
  Dividends Paid Per
    Share.............   $0.96 7/8    $0.96 7/8    $0.96 7/8   $0.968 7/8    $0.96 7/8    $0.96 7/8    $0.96 7/8    $0.96 7/8
  Market Price -- $
    Per Share (OTC)
    (High/Low)........  52.50/52.50  51.75/50.50  53.13/53.13  54.50/52.50  48.25/47.50  50.63/48.50  52.63/52.63  52.50/52.50
</TABLE>
 
- -------------------------
NYSE -- New York Stock Exchange
 
OTC -- Over-the-Counter market quotations provided by National Quotation Bureau,
Inc. High and low bid quotations are not available for the 7.75% Series.
 
   
     Dividends for a Series of Preferred are payable when, as and if declared by
IPC's Board of Directors at the rate per annum included in such title of the
Series of Preferred listed on the front cover of this Offer to Purchase and
Proxy Statement. The May 1998 Dividend has been declared on each Series of
Preferred, payable May 1, 1998 to holders of record as of the close of business
on April 9, 1998. A tender and purchase of Shares pursuant to the Offer will not
deprive such Preferred Shareholder of his or her right to receive the May 1998
Dividend on his or her Shares, regardless of when such tender is made. Tendering
Preferred Shareholders will be entitled to any dividends accrued and unpaid
prior to, but not including, the Payment Date in respect of any later dividend
periods (or any portion thereof). The payment of the May 1998 Dividend
    
 
                                       26
<PAGE>   27
 
will be made separately from payments for Shares tendered in the Offer or the
payment of any Special Cash Payments.
 
                    CERTAIN FEDERAL INCOME TAX CONSEQUENCES
 
     In the opinion of Schiff Hardin & Waite, counsel to ILN and IPC, the
following summary describes the principal United States federal income tax
consequences of sales of Shares pursuant to the Offer and the receipt of Special
Cash Payments in connection with the adoption of the Proposed Amendment. This
summary is based on the Internal Revenue Code of 1986, as amended to the date
hereof (the "Code"), administrative pronouncements, judicial decisions and
existing and proposed Treasury Regulations, changes to any of which subsequent
to the date of this Offer to Purchase and Proxy Statement may adversely affect
the tax consequences described herein, possibly on a retroactive basis. This
summary is addressed to Preferred Shareholders who hold Shares as capital assets
within the meaning of Section 1221 of the Code. This summary does not discuss
all of the tax consequences that may be relevant to a Preferred Shareholder in
light of such Preferred Shareholder's particular circumstances or to Preferred
Shareholders subject to special rules (including certain financial institutions,
tax-exempt organizations, insurance companies, dealers in securities or
currencies, and Preferred Shareholders who are not citizens or residents of the
United States). Preferred Shareholders should consult their tax advisors with
regard to the application of the United States federal income tax laws to their
particular situations as well as any tax consequences arising under the laws of
any state, local or foreign taxing jurisdiction.
 
   
     As used herein, the term "United States Holder" means an owner of a Share
that is for United States federal income tax purposes (i) a citizen or resident
of the United States; (ii) a corporation, partnership or other entity created or
organized in or under the laws of the United States or of any political
subdivision thereof; (iii) an estate, or for taxable years beginning on or
before December 31, 1996, in general, any trust, the income of which is subject
to United States federal income taxation regardless of its source; or (iv) for
taxable years beginning after December 31, 1996, any trust if a court within the
United States is able to exercise primary supervision over the administration of
such trust and one or more United States fiduciaries have the authority to
control all substantial decisions of such trust. A "Non-United States Holder" is
a Preferred Shareholder that is not a United States Holder.
    
 
TAX CONSIDERATIONS FOR TENDERING PREFERRED SHAREHOLDERS
 
     Characterization of the Sale. A sale of Shares by a Preferred Shareholder
pursuant to the Offer will be a taxable transaction for Federal income tax
purposes.
 
     United States Holders. A United States Holder will recognize gain or loss
equal to the difference between the tax basis of such Holder's Shares and the
amount of cash received in exchange therefor. A United States Holder's gain or
loss will be long-term capital gain or loss if the holding period for the Shares
is more than one year as of the date of the sale of such Shares. The excess of
net long-term capital gains over net short-term capital losses is taxed at a
lower rate than ordinary income for certain non-corporate taxpayers. Capital
gain on Shares held by non-corporate taxpayers for more than eighteen months on
the date of the sale of such Shares will be subject to a reduced tax rate. The
distinction between capital gain or loss and ordinary income or loss is also
relevant for purposes of, among other things, limitations on the deductibility
of capital losses.
 
     Non-United States Holders. Any gain realized upon the sale of Shares by a
Non-United States Holder pursuant to the Offer generally will not be subject to
United States Federal income tax unless (i) such gain is effectively connected
with a trade or business in the United States of the Non-United States Holder,
or (ii) in the case of a Non-United States Holder who is an individual, such
individual is present in the United States for 183 days or more in the taxable
year of such sale and certain other conditions are met.
 
     A Non-United States Holder described in clause (i) above will be taxed on
the net gain derived from the sale at regular graduated United States Federal
income tax rates. If a Non-United States Holder that is a foreign corporation
falls under clause (i) above, it may also be subject to an additional "branch
profits tax" at a 30% rate (or such lower rate as may be specified by an
applicable income tax treaty). Unless an applicable
 
                                       27
<PAGE>   28
 
tax treaty provides otherwise, an individual Non-United States Holder described
in clause (ii) above will be subject to a flat 30% tax on the gain derived from
the sale, which may be offset by United States capital losses (notwithstanding
the fact that the individual is not considered a resident of the United States).
 
TAX CONSIDERATIONS FOR NON-TENDERING PREFERRED SHAREHOLDERS
 
   
     Preferred Shareholders will not recognize any taxable gain or loss with
respect to the Shares as a result of the modification of the Articles by the
Proposed Amendment.
    
 
   
     United States Holders. There is no direct authority concerning the Federal
income tax consequences of the receipt of Special Cash Payments. IPC will, for
information reporting purposes, treat Special Cash Payments as ordinary
non-dividend income to recipient United States Holders.
    
 
     Non-United States Holders. IPC will treat Special Cash Payments paid to a
Non-United States Holder of Shares as subject to withholding of United States
Federal income tax at a 30% rate. However, Special Cash Payments that are
effectively connected with the conduct of a trade or business by the Non-United
States Holder within the United States are not subject to the withholding tax
(provided such Non-United States Holder provides two originals of Internal
Revenue Service ("IRS") Form 4224 stating that such Special Cash Payments are so
effectively connected), but instead are subject to United States Federal income
tax on a net income basis at applicable graduated individual or corporate rates.
Any such effectively connected Special Cash Payments received by a foreign
corporation may, under certain circumstances, be subject to an additional
"branch profits tax" at a 30% rate (or such lower rate as may be specified by an
applicable income tax treaty). A Non-United States Holder of Shares eligible for
a reduced rate of United States withholding tax pursuant to an income tax treaty
may obtain a refund of any excess amounts withheld by filing an appropriate
claim for refund with the IRS.
 
BACKUP WITHHOLDING
 
   
     ANY PREFERRED SHAREHOLDER WHO FAILS TO COMPLETE AND SIGN THE SUBSTITUTE
FORM W-9 THAT IS INCLUDED IN THE APPLICABLE LETTER OF TRANSMITTAL AND PROXY OR
THE APPLICABLE SEPARATE PROXY (OR, IN THE CASE OF A FOREIGN PREFERRED
SHAREHOLDER, FORM W-8 OBTAINABLE FROM THE DEPOSITARY) MAY BE SUBJECT TO A
REQUIRED FEDERAL INCOME TAX BACKUP WITHHOLDING OF 31% OF (1) IN THE CASE OF A
TENDERING PREFERRED SHAREHOLDER, THE GROSS AMOUNT PAYABLE TO SUCH PREFERRED
SHAREHOLDER IN EXCHANGE FOR THE SHARES OR (2) IN THE CASE OF A NON-TENDERING
PREFERRED SHAREHOLDER, IN THE EVENT THE PROPOSED AMENDMENT IS ADOPTED AT THE
SPECIAL MEETING, THE SPECIAL CASH PAYMENT. To prevent backup United States
Federal income tax withholding with respect to the purchase price of Shares
purchased pursuant to the Offer or the Special Cash Payment, a United States
Holder must provide the Depositary with the Preferred Shareholder's correct
taxpayer identification number and certify that the Preferred Shareholder is not
subject to backup withholding of Federal income tax by completing the Substitute
Form W-9 included in the applicable Letter of Transmittal and Proxy or the
Separate Proxy. Certain Preferred Shareholders (including, among others, all
corporations and certain foreign shareholders) are exempt from backup
withholding. For a corporate United States Holder to qualify for such exemption,
such Preferred Shareholder must provide the Depositary with a properly completed
and executed Substitute Form W-9 attesting to its exempt status. In order for a
foreign Preferred Shareholder to qualify as an exempt recipient, the foreign
holder must submit a Form W-8, Certificate of Foreign Status, signed under
penalties of perjury, attesting to that Preferred Shareholder's exempt status. A
copy of Form W-8 may be obtained from the Depositary.
    
 
     Unless a Preferred Shareholder provides the appropriate certification,
under the applicable law and regulations concerning "backup withholding" of
Federal United States income tax, the Depositary will be required to withhold,
and will withhold, 31% of the gross proceeds otherwise payable to such Preferred
Shareholder or other payee. The amount of any backup withholding from a payment
to a Preferred Shareholder will be allowed as a credit against such Preferred
Shareholder's United States federal income tax
 
                                       28
<PAGE>   29
 
liability and may entitle such Preferred Shareholder to a refund, provided that
the required information is furnished to the IRS. However, backup withholding is
not required for amounts subject to withholding discussed above under "Tax
Consideration for Tendering Preferred Shareholders -- Non-United States Holders"
and "Tax Considerations for Non-Tendering Preferred Shareholders -- Non-United
States Holders."
 
                           SOURCE AND AMOUNT OF FUNDS
 
   
     Assuming that ILN purchases all outstanding Shares pursuant to the Offer,
the total amount required by ILN to purchase such Shares will be approximately
$  million, exclusive of the May 1, 1998 Dividend, fees and other expenses. ILN
intends to fund the Offer through borrowings under its $150 million Credit
Agreement, dated as of June 12, 1996, as amended on June 28, 1996, with various
financial institutions and CIBC Inc., as Administrative Agent (the "Credit
Agreement"). The Credit Agreement terminates, unless extended, on June 11, 1998.
The Credit Agreement permits ILN to borrow funds at floating interest rates
determined by reference to the federal funds rate published by the Federal
Reserve Bank of New York or, for certain fixed periods, at fixed interest rates
determined by the interbank Eurodollar market. ILN's borrowings under the Credit
Agreement are expected to be repaid through funds received from IPC, which
expects to derive its funds from internally generated funds or the issuance of
short-term debt.
    
 
               TRANSACTIONS AND AGREEMENTS CONCERNING THE SHARES
 
     Each of ILN and IPC has been advised by its directors and executive
officers that no directors or executive officers of the respective companies own
any Shares. Based upon the companies' records and upon information provided to
each company by its directors and executive officers, neither company nor, to
the knowledge of either, any of their subsidiaries, affiliates, directors or
executive officers, or associates of the foregoing, or any of their pension,
profit-sharing or retirement plans, has engaged in any transactions involving
Shares during the 60 business days preceding the date hereof. Neither company
nor, to the knowledge of either, any of its directors or executive officers or
an associate of the foregoing is a party to any contract, arrangement,
understanding or relationship relating directly or indirectly to the Offer with
any other person or entity with respect to any securities of IPC.
 
     IPC purchased Shares on the open market on the dates and at the prices
shown below in fiscal 1996 and 1997.
 
<TABLE>
<CAPTION>
                           NUMBER OF   RANGE OF PURCHASE PRICES   AVERAGE PURCHASE
SERIES    FISCAL QUARTER    SHARES            (HIGH/LOW)          PRICE PER SHARE
- ------    --------------   ---------   ------------------------   ----------------
<C>      <S>               <C>         <C>                        <C>
4.08%    3rd Quarter 1997    16,710        $36.520/$35.000            $35.606
4.20%    3rd Quarter 1997    12,280          37.590/34.841             35.762
4.26%    3rd Quarter 1997    14,000          36.059/36.059             36.059
4.42%    3rd Quarter 1997    15,600          37.413/37.413             37.413
4.70%    3rd Quarter 1997    24,000          38.989/38.989             38.989
7.75%    3rd Quarter 1996   135,100          52.750/52.125             52.421
</TABLE>
 
                  FEES AND EXPENSES ASSOCIATED WITH THE OFFER
 
     Dealer Manager, Depositary and Information Agent Fees. Donaldson, Lufkin &
Jenrette Securities Corporation will act as the Dealer Manager for ILN in
connection with the Offer. ILN has agreed to pay the Dealer Manager a fee of
$     per Share for any Shares tendered, accepted for payment and paid for
pursuant to the Offer and to pay the Dealer Manager a fee of $     per Share for
any Shares that are not tendered pursuant to the Offer but which vote in favor
of the Proposed Amendment, provided that the Proposed Amendment is adopted at
the Special Meeting. The Dealer Manager will also be reimbursed by ILN for its
reasonable out-of-pocket expenses, including attorneys' fees, and will be
indemnified against certain liabilities, including certain liabilities under the
federal securities laws, in connection with the Offer. The Dealer
 
                                       29
<PAGE>   30
 
   
Manager has rendered, is currently rendering and is expected to continue to
render various investment banking and other advisory services to ILN and IPC.
The Dealer Manager has received, and will continue to receive, customary
compensation from ILN and IPC for such services. ILN has retained First Chicago
Trust Company of New York as Depositary and MacKenzie Partners, Inc. as
Information Agent in connection with the Offer. The Depositary and Information
Agent will receive reasonable and customary compensation for their services and
will also be reimbursed for reasonable out-of-pocket expenses, including
attorneys' fees. ILN has agreed to indemnify the Depositary and Information
Agent against certain liabilities, including certain liabilities under the
federal securities laws, in connection with the Offer. Neither the Depositary
nor the Information Agent has been retained to make solicitations or
recommendations in connection with the Offer.
    
 
   
     Solicited Tender Fees; Separate Fees. Pursuant to Instruction 10 of the
accompanying Letter of Transmittal and Proxy, ILN will pay a solicitation fee of
$     per Share that is tendered, accepted for payment and paid for pursuant to
the Offer in transactions for beneficial owners of fewer than           Shares
and a solicitation fee of $     per Share for transactions for beneficial owners
of           or more Shares; provided that solicitation fees payable in
transactions for beneficial owners of           or more Shares shall be paid 80%
to the Dealer Manager and 20% to any designated Soliciting Dealer (which may be
the Dealer Manager). If the Proposed Amendment is adopted at the Special
Meeting, IPC will pay to each designated Soliciting Dealer a separate fee of
$     per Share for Shares that are not tendered pursuant to the Offer but which
are voted in favor of the Proposed Amendment in transactions for beneficial
owners of fewer than           Shares and a separate fee of $     per Share for
Shares that are not tendered pursuant to the Offer but which are voted in favor
of the Proposed Amendment in transactions for beneficial owners of        or
more Shares; provided that the separate fee payable in transactions for
beneficial owners of           or more Shares shall be paid 80% to the Dealer
Manager and 20% to any designated Soliciting Dealer (which may be the Dealer
Manager). A designated Soliciting Dealer is an entity obtaining the tender or
proxy, if the Letter of Transmittal and Proxy or the Separate Proxy, as the case
may be, shall include its name and it is (i) any broker or dealer in securities,
including a Dealer Manager in its capacity as a dealer or broker, which is a
member in good standing of any national securities exchange or of the NASD, (ii)
any foreign broker or dealer not eligible for membership in the NASD which
agrees to conform to the NASD's Rules of Fair Practice in soliciting tenders
outside the United States to the same extent as though it were an NASD member,
or (iii) any bank or trust company.
    
 
   
     No solicitation fee or separate fee (other than solicitation fees payable
to the Dealer Manager as provided above) shall be payable to a Soliciting Dealer
with respect to the tender of Shares or the vote of Shares by a holder unless
the Letter of Transmittal and Proxy or the Separate Proxy accompanying such
tender or vote, as the case may be, designates such Soliciting Dealer. No
solicitation fee or separate fee shall be payable to a Soliciting Dealer in
respect of Shares registered in the name of such Soliciting Dealer unless such
Shares are held by such Soliciting Dealer as nominee and such Shares are being
tendered or voted for the benefit of one or more beneficial owners identified on
the Letter of Transmittal and Proxy or the Separate Proxy or on the Notice of
Solicited Tenders or Notice of Solicited Votes. No solicitation fee or separate
fee shall be payable to a Soliciting Dealer if such Soliciting Dealer is
required for any reason to transfer the amount of such fee to a depositing
holder (other than itself). No solicitation fee shall be paid to a Soliciting
Dealer with respect to Shares tendered for such Soliciting Dealer's own account
and no separate fee shall be paid to a Soliciting Dealer with respect to Shares
voted for such Soliciting Dealer's own account. A Soliciting Dealer shall not be
entitled to a solicitation fee or a separate fee for Shares beneficially owned
by such Soliciting Dealer. No broker, dealer, bank, trust company or fiduciary
shall be deemed to be the agent of ILN, IPC, the Depositary, the Dealer Manager
or the Information Agent for purposes of the Offer.
    
 
     Soliciting Dealers will include any of the organizations described in
clauses (i), (ii) and (iii) above even when the activities of such organizations
in connection with the Offer consist solely of forwarding to clients materials
relating to the Offer, including the Letter of Transmittal and Proxy and
tendering Shares as directed by beneficial owners thereof. No Soliciting Dealer
is required to make any recommendation to holders of Shares as to whether to
tender or refrain from tendering in the Offer. No assumption is made, in making
payment to any Soliciting Dealer, that its activities in connection with the
Offer included any activities other
 
                                       30
<PAGE>   31
 
than those described above, and for all purposes noted in all materials relating
to the Offer, the term "solicit" shall be deemed to mean no more than
"processing shares tendered" or "forwarding to customers materials regarding the
Offer."
 
     Stock Transfer Taxes. ILN will pay all stock transfer taxes, if any,
payable on account of the acquisition of Shares by ILN pursuant to the Offer,
except in certain circumstances where special payment or delivery procedures are
utilized pursuant to Instruction 6 of the accompanying Letter of Transmittal and
Proxy.
 
     Estimated Expenses. Assuming that all Shares of each Series of Preferred
are tendered and purchased by ILN pursuant to the Offer, it is estimated that
the expenses incurred by ILN in connection with the Offer will be as
approximately set forth below. ILN will be responsible for paying all such
expenses.
 
<TABLE>
<S>                                                             <C>
Dealer Manager Fees.........................................    $
Depositary and Information Agent Fees.......................    $
Solicitation Fees...........................................    $
Printing and Mailing Fees...................................    $
Filing Fees.................................................    $
Legal and Miscellaneous.....................................    $
                                                                -----------
     Total..................................................    $
                                                                ===========
</TABLE>
 
                   CERTAIN INFORMATION REGARDING ILN AND IPC;
                           INCORPORATION BY REFERENCE
 
     ILN is a holding company organized in Illinois in May 1994, which conducts
substantially all of its business through its subsidiaries. It has three
principal operating subsidiaries: IPC, organized in May 1923, is a combination
electric and gas utility; Illinova Generating Company, organized in October
1992, is an independent power company which invests in energy supply projects
and competes in the independent power market worldwide; and Illinova Energy
Partners, Inc., organized in July 1994, is in the business of (i) developing and
marketing energy-related services to the unregulated energy market in the United
States and (ii) brokering and marketing electric power and gas to various
customers. IPC's financial position and results of operations are currently the
principal factors affecting ILN's consolidated financial position and results of
operation.
 
     IPC is engaged in the generation, transmission, distribution and sale of
electric energy and the distribution, transportation and sale of natural gas in
the State of Illinois. Its service area is a widely diversified industrial and
agricultural area comprising approximately 15,000 square miles in northern,
central and southern Illinois. Electric service is provided at retail to 310
incorporated municipalities, adjacent suburban and rural areas and numerous
unincorporated municipalities. Gas service is provided to 257 incorporated
municipalities, adjacent suburban areas and numerous unincorporated
municipalities. The larger cities served include Decatur, East St. Louis (gas
only), Champaign, Danville, Belleville, Granite City, Bloomington (electric
only), Galesburg, Urbana and Normal (electric only).
 
     ILN and IPC are subject to the informational requirements of the Exchange
Act and in accordance therewith file reports and other information with the SEC.
Such reports and other information may be inspected and copied at the public
reference facilities maintained by the SEC at 450 Fifth Street, N.W.,
Washington, D.C. 20549; 500 West Madison Street, Suite 1400, Chicago, Illinois
60661-2511; and Seven World Trade Center, Suite 1300, New York, New York 10048.
Copies of such material can be obtained from the Public Reference Section of the
SEC, 450 Fifth Street, N.W., Washington D.C. 20549 at prescribed rates. The SEC
maintains a Web site at http://www.sec.gov containing reports, proxy and
information statements and other information regarding registrants that file
electronically with the SEC, including ILN and IPC. Reports, proxy materials and
other information about ILN and IPC are also available at the offices of the
NYSE, 20 Broad Street, New York, New York 10005. In connection with the Offer,
ILN has filed an Issuer Tender Offer Statement on Schedule 13E-4 and a Rule
13e-3 Transaction Statement on Schedule 13E-3 with the SEC that includes certain
additional information relating to the Offer. ILN's Schedule 13E-4 and
 
                                       31
<PAGE>   32
 
Schedule 13E-3 will not be available at the SEC's regional offices. IPC has
filed this Offer to Purchase and Proxy Statement (and related documents) with
the SEC pursuant to Rule 14a-6 of the Exchange Act.
 
                 SUMMARY OF CONSOLIDATED FINANCIAL INFORMATION;
                           INCORPORATION BY REFERENCE
 
     Set forth below is certain consolidated historical financial information of
IPC and its subsidiaries. The historical financial information (other than the
ratios of earnings to fixed charges) was derived from the audited consolidated
financial statements included in IPC's Annual Report on Form 10-K for the year
ended December 31, 1997.
 
CONDENSED INCOME STATEMENT DATA:
 
<TABLE>
<CAPTION>
                                                                YEAR ENDED DECEMBER 31,
                                                              ---------------------------
                                                                 1997             1996
                                                                 ----             ----
                                                              (THOUSANDS, EXCEPT RATIOS)
<S>                                                           <C>              <C>
Operating Revenues..........................................  $ 1,773.9        $ 1,688.7
Operating Income............................................      278.7            361.4
Allowance for Borrowed Funds Use During Construction........       (5.0)            (6.5)
Interest Expense............................................      128.7            133.0
Net Income (Loss)...........................................      (44.2)           228.6
  Less-Preferred Dividend Requirement.......................       21.5             22.3
  Plus -- Carrying Amount Over (Under) Consideration Paid
     for Redeemed Preferred Stock of Subsidiary.............        0.2             (0.7)
Net Income (Loss) Applicable to Common Stock................      (65.5)           205.6
Ratio of Earnings to Fixed Charges..........................        1.24             3.40
</TABLE>
 
                                       32
<PAGE>   33
 
CONDENSED BALANCE SHEET DATA (AT END OF PERIOD):
 
<TABLE>
<CAPTION>
                                                                   DECEMBER 31,
                                                              -----------------------
                                                                1997           1996
                                                                ----           ----
                                                                    (MILLIONS)
<S>                                                           <C>            <C>
Assets
  Plant and property........................................  $7,353.4       $6,981.5
  Less -- accumulated depreciation..........................   2,808.1        2,419.7
                                                              --------       --------
     Net utility plant......................................   4,545.3        4,561.8
  Nuclear fuel under capital lease..........................     133.0          101.7
  Investments and other assets..............................       5.9           14.5
  Current assets............................................     414.8          444.3
  Deferred charges..........................................     192.5          446.2
                                                              --------       --------
       Total................................................  $5,291.5       $5,568.5
                                                              ========       ========
Capital and Liabilities
  Common stock..............................................  $1,417.3       $1,416.4
  Treasury stock............................................    (207.7)         (86.2)
  Retained earnings.........................................      89.5          245.9
  Preferred stock...........................................      57.1           96.2
  Mandatorily redeemable preferred stock....................     197.0          197.0
  Long-term debt............................................   1,617.5        1,636.4
                                                              --------       --------
       Total capitalization.................................   3,170.7        3,505.7
  Current liabilities.......................................     729.1          655.5
  Deferred credits and other noncurrent liabilities.........   1,391.7        1,407.3
                                                              --------       --------
       Total................................................  $5,291.5       $5,568.5
                                                              ========       ========
</TABLE>
 
     The financial statements of IPC and related information included in its
Annual Report on Form 10-K for the year ended December 31, 1997, and its Current
Reports on Form 8-K, dated February 13, 1998 and January 21, 1998, each as filed
with the SEC, are hereby incorporated by reference. All documents subsequently
filed by IPC pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act
after the date of this Offer to Purchase and Proxy Statement and prior to the
Expiration Date (or any extension thereof) shall be deemed to be incorporated by
reference in this Offer to Purchase and Proxy Statement and to be a part hereof
from the date of filing of such documents. Any statement contained in a document
incorporated or deemed to be incorporated by reference herein shall be deemed to
be modified or superseded for purposes of this Offer to Purchase and Proxy
Statement to the extent that a statement contained herein or in any other
subsequently filed documents which is deemed to be incorporated by reference
herein modifies or supersedes such statement. Any such statement so modified or
superseded shall not be deemed, except as so modified or superseded, to
constitute a part of this Offer to Purchase and Proxy Statement.
 
     IPC will provide without charge to each person to whom a copy of this Offer
to Purchase and Proxy Statement has been delivered, on the written or oral
request of any such person, a copy of any or all of the documents described
above which have been incorporated by reference in this Offer to Purchase and
Proxy Statement, other than exhibits to such documents. Written requests for
copies of such documents should be addressed to the Shareholder Services
Department, Illinova Corporation at 500 South 27th Street, Decatur, Illinois
62525 (telephone (800) 800-8220) or at its Website on the World Wide Web at
http://www.illinova.com. The information relating to ILN and IPC contained in
this Offer to Purchase and Proxy Statement does not purport to be comprehensive
and should be read together with the information contained in the documents
incorporated by reference.
 
                                       33
<PAGE>   34
 
                             SHAREHOLDER PROPOSALS
 
     If a shareholder intends to present a proposal at the 1999 Annual Meeting
of Shareholders of IPC, the proposal must be received by the Corporate Secretary
of IPC not later than November 12, 1998 for inclusion in IPC's proxy or
information statement and form of proxy, if applicable.
 
                                 MISCELLANEOUS
 
     The Offer is not being made to, nor will ILN accept tenders from, owners of
Shares in any jurisdiction in which the Offer or its acceptance would not be in
compliance with the laws of such jurisdiction. ILN is not aware of any
jurisdiction where the making of the Offer or the tender of Shares would not be
in compliance with applicable law. If ILN becomes aware of any jurisdiction
where the making of the Offer or the tender of Shares is not in compliance with
any applicable law, ILN will make a good faith effort to comply with such law.
If, after such good faith effort, ILN cannot comply with such law, the Offer
will not be made to (nor will tenders be accepted from or on behalf of) the
owners of Shares residing in such jurisdiction. In any jurisdiction in which the
securities, blue sky or other laws require the Offer to be made by a licensed
broker or dealer, the Offer will be deemed to be made on ILN's behalf by one or
more registered brokers or dealers licensed under the laws of such jurisdiction.
 
                                            ILLINOVA CORPORATION
                                            ILLINOIS POWER COMPANY
 
   
            , 1998
    
 
                                       34
<PAGE>   35
 
                                                                    ATTACHMENT A
 
                          ILLINOIS DISSENTERS' RIGHTS
 
  SECTION 11.65. RIGHT TO DISSENT.
 
     (a) A shareholder of a corporation is entitled to dissent from, and obtain
payment for his or her shares in the event of any of the following corporate
actions:
 
          (1) consummation of a plan of merger or consolidation or a plan of
     share exchange to which the corporation is a party if
 
             (i) shareholder authorization is required for the merger or
        consolidation or the share exchange by Section 11.20 or the articles of
        incorporation or
 
             (ii) the corporation is a subsidiary that is merged with its parent
        or another subsidiary under Section 11.30;
 
          (2) consummation of sale, lease or exchange of all, or substantially
     all, of the property and assets of the corporation other than in the usual
     and regular course of business;
 
          (3) an amendment of the articles of incorporation that materially and
     adversely affects rights in respect of a dissenter's shares because it:
 
             (i) alters or abolishes a preferential right of such shares;
 
             (ii) alters or abolishes a right in respect of redemption,
        including a provision respecting a sinking fund for the redemption or
        repurchase, of such shares;
 
             (iii) in the case of a corporation incorporated prior to January 1,
        1982, limits or eliminates cumulative voting rights with respect to such
        shares; or
 
          (4) any other corporate action taken pursuant to a shareholder vote if
     the articles of incorporation, by-laws, or a resolution of the board of
     directors provide that shareholders are entitled to dissent and obtain
     payment for their shares in accordance with the procedures set forth in
     Section 11.70 or as may be otherwise provided in the articles, by-laws or
     resolution.
 
     (b) A shareholder entitled to dissent and obtain payment for his or her
shares under this Section may not challenge the corporate action creating his or
her entitlement unless the action is fraudulent with respect to the shareholder
or the corporation or constitutes a breach of a fiduciary duty owed to the
shareholder.
 
     (c) A record owner of shares may assert dissenters' rights as to fewer than
all the shares recorded in such person's name only if such person dissents with
respect to all shares beneficially owned by any one person and notifies the
corporation in writing of the name and address of each person on whose behalf
the record owner asserts dissenters' rights. The rights of a partial dissenter
are determined as if the shares as to which dissent is made and other shares
were recorded in the names of different shareholders. A beneficial owner of
shares who is not the record owner may assert dissenters' rights as to shares
held on such person's behalf only if the beneficial owner submits to the
corporation the record owner's written consent to the dissent before or at the
same time the beneficial owner asserts dissenters' rights.
 
  SECTION 11.70. PROCEDURE TO DISSENT.
 
     (a) If the corporate action giving rise to the right to dissent is to be
approved at a meeting of shareholders, the notice of meeting shall inform the
shareholders of their right to dissent and the procedure to dissent. If, prior
to the meeting, the corporation furnishes to the shareholders material
information with respect to the transaction that will objectively enable a
shareholder to vote on the transaction and to determine whether or not to
exercise dissenters' rights, a shareholder may assert dissenters' rights only if
the shareholder delivers to the corporation before the vote is taken a written
demand for payment for his or her shares if the proposed action is consummated,
and the shareholder does not vote in favor of the proposed action.
 
                                       A-1
<PAGE>   36
 
     (b) If the corporate action giving rise to the right to dissent is not to
be approved at a meeting of shareholders, the notice to shareholders describing
the action taken under Section 11.30 or Section 7.10 shall inform the
shareholders of their right to dissent and the procedure to dissent. If, prior
to or concurrently with the notice, the corporation furnishes to the
shareholders material information with respect to the transaction that will
objectively enable a shareholder to determine whether or not to exercise
dissenters' rights, a shareholder may assert dissenters' rights only if he or
she delivers to the corporation within 30 days from the date of mailing the
notice a written demand for payment for his or her shares.
 
     (c) Within 10 days after the date on which the corporate action giving rise
to the right to dissent is effective or 30 days after the shareholder delivers
to the corporation the written demand for payment, whichever is later, the
corporation shall send each shareholder who has delivered a written demand for
payment a statement setting forth the opinion of the corporation as to the
estimated fair value of the shares, the corporation's latest balance sheet as of
the end of a fiscal year ending not earlier than 16 months before the delivery
of the statement, together with the statement of income for that year and the
latest available interim financial statements, and either a commitment to pay
for the shares of the dissenting shareholder at the estimated fair value thereof
upon transmittal to the corporation of the certificate or certificates, or other
evidence of ownership, with respect to the shares, or instructions to the
dissenting shareholder to sell his or her shares within 10 days after delivery
of the corporation's statement to the shareholder. The corporation may instruct
the shareholder to sell only if there is public market for the shares at which
the shares may be readily sold. If the shareholder does not sell within that 10
day period after being so instructed by the corporation, for purposes of this
Section the shareholder shall be deemed to have sold his or her shares at the
average closing price of the shares, if listed on a national exchange, or the
average of the bid and asked price with respect to the shares quoted by a
principal market maker, if not listed on a national exchange, during that 10 day
period.
 
     (d) A shareholder who makes written demand for payment under this Section
retains all other rights of a shareholder until those rights are canceled or
modified by the consummation of the proposed corporate action. Upon consummation
of that action, the corporation shall pay to each dissenter who transmits to the
corporation the certificate or other evidence of ownership of the shares the
amount the corporation estimates to be the fair value of the shares, plus
accrued interest, accompanied by a written explanation of how the interest was
calculated.
 
     (e) If the shareholder does not agree with the opinion of the corporation
as to the estimated fair value of the shares or the amount of interest due, the
shareholder, within 30 days from the delivery of the corporation's statement of
value, shall notify the corporation in writing of the shareholder's estimated
fair value and amount of interest due and demand payment for the difference
between the shareholder's estimate of fair value and interest due and the amount
of the payment by the corporation or the proceeds of sale by the shareholder,
whichever is applicable because of the procedure for which the corporation opted
pursuant to subsection (c).
 
     (f) If, within 60 days from delivery to the corporation of the shareholder
notification of estimate of fair value of the shares and interest due, the
corporation and the dissenting shareholder have not agreed in writing upon the
fair value of the shares and interest due, the corporation shall either pay the
difference in value demanded by the shareholder, with interest, or file a
petition in the circuit court of the county in which either the registered
office or the principal office of the corporation is located, requesting the
court to determine the fair value of the shares and interest due. The
corporation shall make all dissenters, whether or not residents of this State,
whose demands remain unsettled parties to the proceeding as an action against
their shares and all parties shall be served with a copy of the petition.
Nonresidents may be served by registered or certified mail or by publication as
provided by law. Failure of the corporation to commence an action pursuant to
this Section shall not limit or affect the right of the dissenting shareholders
to otherwise commence an action as permitted by law.
 
     (g) The jurisdiction of the court in which the proceeding is commenced
under subsection (f) by a corporation is plenary and exclusive. The court may
appoint one or more persons as appraisers to receive evidence and recommend
decision of the question of fair value. The appraisers have the power described
in the order appointing them, or in any amendment to it.
 
                                       A-2
<PAGE>   37
 
     (h) Each dissenter made a party to the proceeding is entitled to judgment
for the amount, if any, by which the court finds that the fair value of his or
her shares, plus interest, exceeds the amount paid by the corporation or the
proceeds of sale by the shareholder, whichever amount is applicable.
 
     (i) The court, in a proceeding commenced under subsection (f), shall
determine all costs of the proceeding, including the reasonable compensation and
expenses of the appraisers, if any, appointed by the court under subsection (g),
but shall exclude the fees and expenses of counsel and experts for the
respective parties. If the fair value of the shares as determined by the court
materially exceeds the amount which the corporation estimated to be the fair
value of the shares or if no estimate was made in accordance with subsection
(c), then all or any part of the costs may be assessed against the corporation.
If the amount which any dissenter estimated to be the fair value of the shares
materially exceeds the fair value of the shares as determined by the court, then
all or any part of the costs may be assessed against that dissenter. The court
may also assess the fees and expenses of counsel and experts for the respective
parties, in amounts the court finds equitable, as follows:
 
          (1) Against the corporation and in favor of any or all dissenters if
     the court finds that the corporation did not substantially comply with the
     requirements of subsections (a), (b), (c), (d), or (f).
 
          (2) Against either the corporation or a dissenter and in favor of any
     other party if the court finds that the party against whom the fees and
     expenses are assessed acted arbitrarily, vexatiously, or not in good faith
     with respect to the rights provided by this Section.
 
     If the court finds that the services of counsel for any dissenter were of
substantial benefit to other dissenters similarly situated and that the fees for
those services should not be assessed against the corporation, the court may
award to that counsel reasonable fees to be paid out of the amounts awarded to
the dissenters who are benefitted. Except as otherwise provided in this Section,
the practice, procedure, judgment and costs shall be governed by the Code of
Civil Procedure.
 
     (j) As used in this Section:
 
          (1) "Fair value", with respect to a dissenter's shares, means the
     value of the shares immediately before the consummation of the corporate
     action to which the dissenter objects excluding any appreciation or
     depreciation in anticipation of the corporate action, unless exclusion
     would be inequitable.
 
          (2) "Interest" means interest from the effective date of the corporate
     action until the date of payment, at the average rate currently paid by the
     corporation on its principal bank loans or, if none, at a rate that is fair
     and equitable under all the circumstances.
 
                                       A-3
<PAGE>   38
 
   
     Facsimile copies of the Letter of Transmittal and Proxy will only be
accepted from Eligible Institutions. Facsimile copies of the Separate Proxy will
be accepted from any holder. The Letter of Transmittal and Proxy and, if
applicable, certificates for Shares or the Separate Proxy should be sent or
delivered by each tendering or voting Preferred Shareholder of IPC or his or her
broker, dealer, bank or trust company to the Depositary at one of its addresses
set forth below.
    
 
                               The Depositary is:
 
                    FIRST CHICAGO TRUST COMPANY OF NEW YORK
 
<TABLE>
<CAPTION>
           By Mail:                           By Hand:                     By Overnight Courier:
           --------                           --------                     ---------------------
<S>                                <C>                                <C>
  First Chicago Trust Company        First Chicago Trust Company        First Chicago Trust Company
          of New York                        of New York                        of New York
   Attn: Tenders & Exchanges          Attn: Tenders & Exchanges          Attn: Tenders & Exchanges
         P.O. Box 2565                c/o The Depository Trust                Suite 4680-CBE
          Suite 4660                Company 55 Water Street, DTC              14 Wall Street
  Jersey City, NJ 07303-2565                     TAD                             8th Floor
                                   Vietnam Veterans Memorial Plaza          New York, NY 10005
                                         New York, NY 10041
</TABLE>
 
                                 By Facsimile:
 
                                 (201) 222-4720
                               or (201) 222-4721
 
                             Confirm by Telephone:
 
                                 (201) 222-4707
 
   
     Any questions or requests for assistance may be directed to the Information
Agent or the Dealer Manager at their respective telephone numbers and addresses
listed below. Requests for additional copies of this Offer to Purchase and Proxy
Statement, the Letter of Transmittal and Proxy, the Separate Proxy or other
tender offer or proxy materials may be directed to the Information Agent, and
such copies will be furnished promptly at the companies' expense. Preferred
Shareholders may also contact their local broker, dealer, commercial bank or
trust company for assistance concerning the Offer.
    
 
                             The Information Agent:
 
                            MACKENZIE PARTNERS, INC.
                                156 Fifth Avenue
                            New York, New York 10010
                           (800) 322-2885 (Toll Free)
                         (212) 929-5500 (Call Collect)
 
                              The Dealer Manager:
 
                          DONALDSON, LUFKIN & JENRETTE
                                SECURITIES CORPORATION
                                277 Park Avenue
                            New York, New York 10172
                           (800) 334-1604 (Toll Free)
                         (212) 892-3351 (Call Collect)
                        Attn: Paul Galant or Jeff Dorst

<PAGE>   1
 
                                                                         IPC   %
 
                        LETTER OF TRANSMITTAL AND PROXY
                                  to Accompany
                Shares of    % Series Cumulative Preferred Stock
                          CUSIP Number 452092 ________
                                       of
                             ILLINOIS POWER COMPANY
              Tendered Pursuant to the Offer to Purchase for Cash
                                       by
                              ILLINOVA CORPORATION
                   dated [Statement Date], for Purchase at a
                       Purchase Price Of $      Per Share
 
       THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 5:00 P.M., NEW YORK
         CITY TIME, ON [EXPIRATION DATE], UNLESS THE OFFER IS EXTENDED.
 
                  To: First Chicago Trust Company of New York
 
<TABLE>
<CAPTION>
           By Mail:                         By Hand:                   By Overnight Courier:
           --------                         --------                   ---------------------
<S>                              <C>                              <C>
First Chicago Trust Company of   First Chicago Trust Company of   First Chicago Trust Company of
           New York                         New York                         New York
   Attn: Tenders & Exchanges        Attn: Tenders & Exchanges        Attn: Tenders & Exchanges
         P.O. Box 2565              c/o The Depository Trust              Suite 4680-CBE
          Suite 4660                         Company                      14 Wall Street
  Jersey City, NJ 07303-2565        55 Water Street, DTC TAD                 8th Floor
                                 Vietnam Veterans Memorial Plaza        New York, NY 10005
                                       New York, NY 10041
</TABLE>
 
  By Facsimile: (201) 222-4720 or 222-4721         Confirm by Telephone: (201)
                                    222-4707
 
   
ILLINOVA CORPORATION ("ILN") WILL NOT BE REQUIRED TO ACCEPT FOR PAYMENT OR PAY
FOR ANY SHARES TENDERED IF THE PROPOSED AMENDMENT IS NOT APPROVED AND ADOPTED AT
THE SPECIAL MEETING. PREFERRED SHAREHOLDERS OF RECORD OF ILLINOIS POWER COMPANY
("IPC") HAVE THE RIGHT TO VOTE FOR THE PROPOSED AMENDMENT REGARDLESS OF WHETHER
THEY TENDER THEIR SHARES. IF THE PROPOSED AMENDMENT IS ADOPTED AT THE SPECIAL
MEETING, IPC WILL MAKE A SPECIAL CASH PAYMENT TO EACH PREFERRED SHAREHOLDER OF
RECORD AS OF [RECORD DATE], PROVIDED THAT THEIR SHARES ARE NOT TENDERED PURSUANT
TO THE OFFER.
    
 
<TABLE>
<S>                                                          <C>              <C>              <C>
- ---------------------------------------------------------------------------------------------------------------
                                        DESCRIPTION OF SHARES TENDERED
- ---------------------------------------------------------------------------------------------------------------
      NAME(S) AND ADDRESS(ES) OF REGISTERED HOLDER(S)
 IF TENDERING SHARES, PLEASE FILL IN EXACTLY AS INFORMATION
APPEARS ON CERTIFICATE(S) (ATTACH ADDITIONAL SIGNED LIST IF    CERTIFICATE     REPRESENTED BY  NUMBER OF SHARES
                         NECESSARY)                             NUMBER(S)*     CERTIFICATE(S)     TENDERED**
- ---------------------------------------------------------------------------------------------------------------
                                                             ==================================================
                                                             --------------------------------------------------
- ---------------------------------------------------------------------------------------------------------------
                       TOTAL NUMBER OF SHARES TENDERED
- ---------------------------------------------------------------------------------------------------------------
   * Need not be completed by shareholders tendering by book-entry transfer.
  ** Unless otherwise indicated, it will be assumed that all Shares represented by any certificates delivered
     to the Depositary are being tendered. See Instruction 4.
- ---------------------------------------------------------------------------------------------------------------
</TABLE>
 
NOTE: IF YOU ARE VOTING AND TENDERING SHARES, USE THE PROXY CONTAINED IN THIS
LETTER OF TRANSMITTAL AND PROXY. IF YOU ARE VOTING BUT NOT TENDERING SHARES, USE
THE SEPARATE [PROXY CARD COLOR] PROXY.
<PAGE>   2
 
NOTE: IF SHARES ARE BEING TENDERED, THE REMAINDER OF THIS LETTER OF TRANSMITTAL
AND PROXY (THE "LETTER OF TRANSMITTAL AND PROXY") MUST BE COMPLETED, INCLUDING,
IF APPLICABLE, THE SUBSTITUTE FORM W-9 BELOW.
 
   
DELIVERY OF THIS LETTER OF TRANSMITTAL AND PROXY TO AN ADDRESS OTHER THAN AS SET
FORTH ABOVE WILL NOT CONSTITUTE A VALID DELIVERY. YOU MUST SIGN THIS LETTER OF
TRANSMITTAL AND PROXY IN THE APPROPRIATE SPACE THEREFOR PROVIDED AND, IF YOU ARE
TENDERING ANY SHARES, COMPLETE THE SUBSTITUTE FORM W-9 SET FORTH BELOW OR A FORM
W-8, AS APPLICABLE. SEE INSTRUCTION 8 AND "IMPORTANT TAX INFORMATION" BELOW.
    
 
DO NOT SEND ANY CERTIFICATES TO DONALDSON, LUFKIN & JENRETTE SECURITIES
CORPORATION, MACKENZIE PARTNERS, INC., ILLINOVA CORPORATION OR ILLINOIS POWER
COMPANY.
 
THE INSTRUCTIONS ACCOMPANYING THIS LETTER OF TRANSMITTAL AND PROXY SHOULD BE
READ CAREFULLY BEFORE THIS LETTER OF TRANSMITTAL AND PROXY IS COMPLETED.
QUESTIONS REGARDING AND REQUESTS FOR COPIES OF THE OFFER TO PURCHASE AND PROXY
STATEMENT OR THIS LETTER OF TRANSMITTAL AND PROXY MAY BE DIRECTED TO MACKENZIE
PARTNERS, INC., THE INFORMATION AGENT, AT 156 FIFTH AVENUE, NEW YORK, NEW YORK
10010 OR TELEPHONE (800) 322-2885 (TOLL FREE).
 
This Letter of Transmittal and Proxy is to be used (a) if certificates are to be
forwarded to the First Chicago Trust Company of New York ("Depositary") or (b)
if delivery of tendered Shares (as defined below) is to be made by book-entry
transfer to the Depositary's account at The Depository Trust Company ("DTC" or
the "Book-Entry Transfer Facility") pursuant to the procedures set forth under
the heading "Terms of the Offer -- Procedure for Tendering Shares" in the Offer
to Purchase and Proxy Statement (as defined below).
 
   
Preferred Shareholders (as defined below) who wish to tender Shares but who
cannot deliver their Shares and all other documents required hereby to the
Depositary by the Expiration Date (as defined in the Offer to Purchase and Proxy
Statement) must tender their Shares pursuant to the guaranteed delivery
procedure set forth under the heading "Terms of the Offer -- Procedure for
Tendering Shares -- Guaranteed Delivery Procedure" in the Offer to Purchase and
Proxy Statement. See Instruction 2. DELIVERY OF DOCUMENTS TO ILN, IPC OR THE
BOOK-ENTRY TRANSFER FACILITY DOES NOT CONSTITUTE A VALID DELIVERY.
    
 
If any of your certificate(s) for Shares have been lost, stolen or destroyed,
please call Illinova Corporation's Shareholder Services Department (800)
800-8220. In addition, you should advise the Shareholder Services Department of
any certificate(s) you have in your possession. You will need to complete an
Affidavit of Loss with respect to the lost certificate(s) (which will be
provided by the Shareholder Services Department) and pay an indemnity bond
premium fee.
 
     THIS LETTER OF TRANSMITTAL AND PROXY IS SOLICITED ON BEHALF OF THE BOARD OF
DIRECTORS OF IPC. The proxy contained herein, when properly executed, will be
voted in the manner directed herein by the undersigned shareholder(s). If no
direction is made, the proxy will be voted for Item 1.
 
   
                                     PROXY
    
 
   
     The undersigned hereby appoints        ,        and        , or any of
them, as proxies, each with the power to appoint his substitute, and hereby
authorizes them to represent and to vote as designated hereunder and in their
discretion with respect to any other business properly brought before the
Special Meeting, all the shares of cumulative preferred stock of Illinois Power
Company ("IPC") which the undersigned is entitled to vote at the Special Meeting
of Shareholders to be held on           , 1998, or any adjournment(s) or
postponement(s) thereof.
    
<PAGE>   3
 
     INDICATE YOUR VOTE BY AN (X). THE BOARD OF DIRECTORS OF IPC RECOMMENDS
VOTING FOR ITEM 1.
 
ITEM 1.
 
     To remove in its entirety ARTICLE V, Section 1, Clause (f)(1) from the
Amended and Restated Articles of Incorporation of IPC (the "Articles"), which
limits IPC's ability to issue or assume unsecured indebtedness.
 
       [ ] FOR                   [ ] AGAINST                  [ ] ABSTAIN
 
   
NOTE: THE SUBSTITUTE FORM W-9 BELOW SHOULD BE COMPLETED TO AVOID BACK-UP
WITHHOLDING ON THE SPECIAL CASH PAYMENT.
    
 
   
     SHARES REPRESENTED BY ALL PROPERLY EXECUTED PROXIES WILL BE VOTED IN
ACCORDANCE WITH INSTRUCTIONS APPEARING IN THIS PROXY. IN THE ABSENCE OF SPECIFIC
INSTRUCTIONS, PROXIES WILL BE VOTED IN ACCORDANCE WITH THE RECOMMENDATIONS OF
THE BOARD OF DIRECTORS OF IPC AND IN THE DISCRETION OF THE PROXY HOLDERS AS TO
ANY OTHER MATTERS THAT MAY PROPERLY COME BEFORE THE SPECIAL MEETING.
    
 
   
     Any holder of Shares held of record on [RECORD DATE] in the name of another
holder must establish to the satisfaction of IPC his or her entitlement to
exercise or transfer this Proxy. This will ordinarily require an assignment by
such record holder in blank, or if not in blank, to and from each successive
transferee, including the holder, with each signature guaranteed by an Eligible
Institution.
    
 
Please check box if you plan to attend the Special Meeting.  [ ]
 
     (BOXES BELOW FOR USE BY ELIGIBLE INSTITUTIONS ONLY)
 
[ ]  CHECK HERE IF TENDERED SHARES ARE BEING DELIVERED BY BOOK-ENTRY TRANSFER TO
     THE DEPOSITARY'S ACCOUNT AT DTC AND COMPLETE THE FOLLOWING:
 
    Name of Tendering Institution:
 
- --------------------------------------------------------------------------------
    Account No.:
    ----------------------------------------------------------------------------
    VOI No.:
    ----------------------------------------------------------------------------
 
[ ]  CHECK HERE IF TENDERED SHARES ARE BEING DELIVERED PURSUANT TO A NOTICE OF
     GUARANTEED DELIVERY PREVIOUSLY SENT TO THE DEPOSITARY AND COMPLETE THE
     FOLLOWING:
 
    Name(s) of tendering shareholder(s):
    -------------------------------------------------------------------------
    Date of Execution of Notice of Guaranteed Delivery:
    ---------------------------------------------------------
    Name of Institution that Guaranteed Delivery:
    ----------------------------------------------------------------
    If delivery is by book-entry transfer:
    Name of Tendering Institution:
 
- --------------------------------------------------------------------------------
    Account No.:
    ----------------------------------------------------------------------------
    VOI No.:
    ----------------------------------------------------------------------------
<PAGE>   4
 
                    NOTE: SIGNATURES MUST BE PROVIDED BELOW
              PLEASE READ THE ACCOMPANYING INSTRUCTIONS CAREFULLY
 
Ladies and Gentlemen:
 
   
     The undersigned hereby tenders to Illinova Corporation, an Illinois
corporation ("ILN"), the shares in the amount set forth in the box above
designated "Description of Shares Tendered" pursuant to ILN's offer to purchase
any and all of the outstanding shares of the series of cumulative preferred
stock of Illinois Power Company, an Illinois corporation, and direct utility
subsidiary of ILN ("IPC"), shown on the first page hereof as to which this
Letter of Transmittal and Proxy is applicable (the "Shares") at the purchase
price per Share shown on the first page hereof plus accrued and unpaid dividends
for the Shares tendered, net to the seller in cash, upon the terms and subject
to the conditions set forth in the Offer to Purchase and Proxy Statement, dated
[STATEMENT DATE] (the "Offer to Purchase and Proxy Statement"), receipt of which
is hereby acknowledged, and in this Letter of Transmittal and Proxy (which as to
the Shares, together with the Offer to Purchase and Proxy Statement, constitutes
the "Offer"). WHILE HOLDERS OF SHARES ("PREFERRED SHAREHOLDERS") WHO WISH TO
TENDER THEIR SHARES PURSUANT TO THE OFFER NEED NOT VOTE IN FAVOR OF THE PROPOSED
AMENDMENT TO IPC'S AMENDED AND RESTATED ARTICLES OF INCORPORATION, AS SET FORTH
IN THE OFFER TO PURCHASE AND PROXY STATEMENT (THE "PROPOSED AMENDMENT"), THE
OFFER IS CONDITIONED UPON THE PROPOSED AMENDMENT BEING APPROVED AND ADOPTED AT
THE SPECIAL MEETING (AS DEFINED IN THE OFFER TO PURCHASE AND PROXY STATEMENT).
See "Proposed Amendment and Proxy Solicitation", "Terms of the Offer --
Extension of Tender Period; Termination; Amendments" and "Terms of the Offer --
Certain Conditions of the Offer" in the Offer to Purchase and Proxy Statement.
    
 
     Subject to, and effective upon, acceptance for payment of and payment for
the Shares tendered herewith in accordance with the terms and subject to the
conditions of the Offer (including, if the Offer is extended or amended, the
terms and conditions of any such extension or amendment), the undersigned hereby
sells, assigns and transfers to, or upon the order of, ILN all right, title and
interest in and to all the Shares that are being tendered hereby and hereby
constitutes and appoints First Chicago Trust Company of New York (the
"Depositary") the true and lawful agent and attorney-in-fact of the undersigned
with respect to such Shares, with full power of substitution (such power of
attorney being an irrevocable power coupled with an interest), to (a) deliver
certificates for such Shares, or transfer ownership of such Shares on the
account books maintained by DTC, together, in any such case, with all
accompanying evidences of transfer and authenticity, to or upon the order of
ILN, (b) present such Shares for registration and transfer on the books of IPC
and (c) receive all benefits and otherwise exercise all rights of beneficial
ownership of such Shares, all in accordance with the terms of the Offer. The
Depositary will act as agent for tendering shareholders for the purpose of
receiving payment from ILN and transmitting payment to tendering shareholders.
 
     The undersigned hereby represents and warrants that the undersigned has
full power and authority to tender, sell, assign and transfer the Shares
tendered hereby and that, when and to the extent the same are accepted for
payment by ILN, ILN will acquire good, marketable and unencumbered title
thereto, free and clear of all liens, restrictions, charges, encumbrances,
conditional sales agreements or other obligations relating to the sale or
transfer thereof, and the same will not be subject to any adverse claims. The
undersigned will, upon request, execute and deliver any additional documents
deemed by the Depositary or ILN to be necessary or desirable to complete the
sale, assignment and transfer of the Shares tendered hereby.
 
     All authority herein conferred or agreed to be conferred shall not be
affected by, and shall survive the death, bankruptcy or incapacity of the
undersigned, and any obligations of the undersigned hereunder shall be binding
upon the heirs, legal representatives, successors, assigns, executors and
administrators of the undersigned. Except as stated in the Offer, this tender is
irrevocable.
 
     The undersigned understands that tenders of Shares pursuant to any one of
the procedures described under the heading "Terms of the Offer -- Procedure for
Tendering Shares" in the Offer to Purchase and Proxy Statement and in the
instructions hereto will constitute the undersigned's acceptance of the terms
and
<PAGE>   5
 
conditions of the Offer, including the undersigned's representation and warranty
that (a) the undersigned has a net long position in the Shares being tendered
within the meaning of Rule 14e-4 promulgated under the Securities Exchange Act
of 1934, as amended, and (b) the tender of such Shares complies with such Rule
14e-4. ILN's acceptance for payment of Shares tendered pursuant to the Offer
will constitute a binding agreement between the undersigned and ILN upon the
terms and subject to the conditions of the Offer.
 
     The undersigned recognizes that, under certain circumstances set forth in
the Offer to Purchase and Proxy Statement, ILN may terminate or amend the Offer
or may not be required to purchase any of the Shares tendered hereby. In either
event, the undersigned understands that certificate(s) for any Shares not
tendered or not purchased will be returned to the undersigned.
 
   
     Unless otherwise indicated in the box below under the heading "Special
Payment Instructions", please issue the check for the purchase price of any
Shares purchased, and/or return any Shares not tendered or not purchased, in the
name(s) of the undersigned (and, in the case of Shares tendered by book-entry
transfer, by credit to the account at the Book-Entry Transfer Facility).
Similarly, unless otherwise indicated in the box below under the heading
"Special Delivery Instructions", please mail the check for the purchase price of
any Shares purchased and/or any certificates for Shares not tendered or not
purchased (and accompanying documents, as appropriate) to the undersigned at the
address shown below the undersigned signature(s). In the event that both
"Special Payment Instructions" and "Special Delivery Instructions" are
completed, please issue the check for the purchase price of any Shares purchased
and/or return any Shares not tendered or not purchased in the name(s) of, and
mail said check and/or any certificates to, the person(s) so indicated. The
undersigned recognizes that ILN has no obligation, pursuant to the "Special
Payment Instructions", to transfer any Shares from the name of the registered
holder(s) thereof if ILN does not accept for payment any of the Shares so
tendered.
    
 
                          SPECIAL PAYMENT INSTRUCTIONS
                        (SEE INSTRUCTIONS 1, 4, 5 AND 7)
To be completed ONLY if the check for the purchase price of Shares purchased
and/or certificates for Shares not tendered or not purchased are to be issued in
the name of someone other than the undersigned.
 
Issue: [ ] check to:
       [ ] certificate(s) to:
Name
- -----------------------------------------------
                                 (Please Print)
 
Address
- ---------------------------------------------
 
- -------------------------------------------------------
 
- -------------------------------------------------------
                               (Include Zip Code)
 
- -------------------------------------------------------
              (Taxpayer Identification or Social Security Number)
 
                         SPECIAL DELIVERY INSTRUCTIONS
                           (SEE INSTRUCTIONS 4 AND 7)
To be completed ONLY if the check for the purchase price of Shares purchased
and/or certificates for Shares not tendered or not purchased are to be mailed to
someone other than the undersigned or to the undersigned at an address other
than that shown below the undersigned's signature(s).
 
Mail: [ ] check to:
       [ ] certificate(s) to:
Name
- -----------------------------------------------
                                 (Please Print)
 
Address
- ---------------------------------------------
 
- -------------------------------------------------------
 
- -------------------------------------------------------
                               (Include Zip Code)
 
- -------------------------------------------------------
              (Taxpayer Identification or Social Security Number)
<PAGE>   6
 
- --------------------------------------------------------------------------------
 
                            SIGNATURE(S) OF OWNER(S)
 
   X
    --------------------------------------------------------------------------
 
   X
    --------------------------------------------------------------------------
 
   Dated:                                                               , 1998
         ---------------------------------------------------------------
 
   Name(s):
           ------------------------------------------------------------------
 
   --------------------------------------------------------------------------
                                (Please Print)
 
   Capacity (Full Title or "Individual," if applicable)
 
   --------------------------------------------------------------------------
 
   --------------------------------------------------------------------------
 
   Addresses:
             ----------------------------------------------------------------
 
   --------------------------------------------------------------------------
                               (Include Zip Code)
 
   Daytime Area Code and Telephone No.:
 
   --------------------------------------------------------------------------
 
        (Must be signed by the registered holder(s) exactly as name(s)
   appear(s) on the stock certificate(s) or on a security position listing or
   by person(s) authorized to become registered holder(s) by certificates and
   documents transmitted herewith. If signature is by a trustee, executor,
   administrator, guardian, attorney-in-fact, officer of a corporation or
   other person acting in a fiduciary or representative capacity, please set
   forth full title and see Instruction 5.)
 
              GUARANTEE OF SIGNATURE(S) (SEE INSTRUCTIONS 1 AND 5)
 
   Authorized Signature:
                        -----------------------------------------------------
 
   Name:
        ---------------------------------------------------------------------
 
   Name of Firm:
                -------------------------------------------------------------
 
   Address of Firm:
                   ----------------------------------------------------------
 
   Area Code and Telephone No.:
                               ----------------------------------------------
 
   Dated:                                                              , 1998
         --------------------------------------------------------------
 
   IF SHARES ARE BEING TENDERED, PLEASE COMPLETE SUBSTITUTE FORM W-9 BELOW
                        OR A FORM W-8, AS APPLICABLE.
- --------------------------------------------------------------------------------
<PAGE>   7
 
                               SOLICITED TENDERS
                              (SEE INSTRUCTION 10)
 
     As provided in Instruction 10, ILN will pay a solicitation fee of $
per Share that is tendered, accepted for payment and paid for pursuant to the
Offer in transactions for beneficial owners of fewer than      Shares and a
solicitation fee of $     per Share for transactions for beneficial owners of
     or more Shares; provided that solicitation fees payable in transactions for
beneficial owners of      or more Shares shall be paid 80% to the Dealer Manager
and 20% to any designated Soliciting Dealer (as defined in Instruction 10)
(which may be the Dealer Manager). However, Soliciting Dealers will not be
entitled to a solicitation fee for Shares beneficially owned by such Soliciting
Dealer.
 
     The above signed represents that the Soliciting Dealer which solicited and
obtained this tender is:
 
Name of Firm:
                                 (Please Print)
 
Name of Individual Broker or Financial Consultant:
                                                     (Please Print)
 
Telephone Number of Broker or Financial Consultant:
 
Identification Number (if known):
 
Address:
 
                                  (Include zip code)
 
     The following to be completed ONLY if customer's Shares held in nominee
name are tendered.
 
<TABLE>
<S>                                                <C>
          NAME OF BENEFICIAL OWNER                          NUMBER OF SHARES TENDERED
</TABLE>
 
                     (ATTACH ADDITIONAL LIST IF NECESSARY)
 
     The acceptance of compensation by such Soliciting Dealer will constitute a
representation by it that (a) it has complied with the applicable requirements
of the Securities Exchange Act of 1934, as amended, and the applicable rules and
regulations thereunder, in connection with such solicitation; (b) it is entitled
to such compensation for such solicitation under the terms and conditions of the
Offer to Purchase and Proxy Statement; (c) in soliciting tenders of Shares, it
has used no solicitation materials other than those furnished by ILN; and (d) if
it is a foreign broker or dealer not eligible for membership in the National
Association of Securities Dealers, Inc. (the "NASD"), it has agreed to conform
to the NASD's Rules of Fair Practice in making solicitations.
 
     The payment of compensation to any Soliciting Dealer is dependent on such
Soliciting Dealer returning a Notice of Solicited Tenders to the Depositary.
 
     THIS LETTER OF TRANSMITTAL AND PROXY IS TO BE USED ONLY FOR THE TENDER OF
SHARES OF THE SERIES OF CUMULATIVE PREFERRED STOCK OF IPC SHOWN ON THE FIRST
PAGE HEREOF. ANY PERSON DESIRING TO TENDER SHARES OF ANY OTHER SERIES OF
CUMULATIVE PREFERRED STOCK FOR WHICH ILN IS MAKING A TENDER OFFER MUST SUBMIT
THE LETTER OF TRANSMITTAL AND PROXY RELATING TO THAT SPECIFIC SERIES.
<PAGE>   8
 
                                  INSTRUCTIONS
 
             FORMING PART OF THE TERMS AND CONDITIONS OF THE OFFER
 
     1. GUARANTEE OF SIGNATURES. Except as otherwise provided below, all
signatures on this Letter of Transmittal and Proxy must be guaranteed by a firm
that is a member in good standing of a registered national securities exchange
or the NASD or by a commercial bank or trust company having an office or
correspondent in the United States which is a participant in an approved
Medallion Signature Guarantee Program (each of the foregoing being referred to
as an "Eligible Institution"). Signatures on this Letter of Transmittal and
Proxy need not be guaranteed (a) if this Letter of Transmittal and Proxy is
signed by the registered holder(s) of the Shares (which term, for purposes of
this document, shall include any participant in one of the Book-Entry Transfer
Facilities whose name appears on a security position listing as the owner of
Shares) tendered herewith and such holder(s) has not completed the box above
under the heading "Special Payment Instructions" or the box above under the
heading "Special Delivery Instructions" on this Letter of Transmittal and Proxy,
or (b) if such Shares are tendered for the account of an Eligible Institution.
See Instruction 5.
 
   
     2. DELIVERY OF LETTER OF TRANSMITTAL AND PROXY AND SHARES. This Letter of
Transmittal and Proxy is to be used if (a) certificates are to be forwarded
herewith, or (b) delivery of Shares is to be made by book-entry transfer
pursuant to the procedures set forth under the heading "Terms of the Offer --
Procedure for Tendering Shares" in the Offer to Purchase and Proxy Statement.
Certificates for all physically delivered Shares, or a confirmation of a
book-entry transfer into the Depositary's account at the Book-Entry Transfer
Facility of all Shares delivered electronically, as well as a properly completed
and duly executed Letter of Transmittal and Proxy, and any other documents
required by this Letter of Transmittal and Proxy, must be received by the
Depositary at one of its addresses set forth on the front page of this Letter of
Transmittal and Proxy on or prior to the Expiration Date (as defined in the
Offer to Purchase and Proxy Statement) with respect to all Shares. Preferred
Shareholders who wish to tender their Shares but cannot deliver their Shares and
all other required documents to the Depositary on or prior to the Expiration
Date must tender their Shares pursuant to the guaranteed delivery procedure set
forth under the heading "Terms of the Offer -- Procedure for Tendering Shares --
Guaranteed Delivery Procedure" in the Offer to Purchase and Proxy Statement.
Pursuant to such procedure: (a) such tender must be made by or through an
Eligible Institution, (b) a properly completed and duly executed Notice of
Guaranteed Delivery in the form provided by ILN (with any required signature
guarantees) must be received by the Depositary on or prior to the applicable
Expiration Date and (c) the certificates for all physically delivered Shares, or
a confirmation of a book-entry transfer into the Depositary's account at the
Book-Entry Transfer Facility of all Shares delivered electronically, together
with a properly completed and duly executed Letter of Transmittal and Proxy and
any other documents required by this Letter of Transmittal and Proxy must be
received by the Depositary by 5:00 p.m. (New York City time) within three New
York Stock Exchange trading days after the date of execution of such Notice of
Guaranteed Delivery, all as provided under the heading "Terms of the Offer --
Procedure for Tendering Shares -- Guaranteed Delivery Procedure" in the Offer to
Purchase and Proxy Statement.
    
 
     THE METHOD OF DELIVERY OF SHARES AND ALL OTHER REQUIRED DOCUMENTS IS AT THE
OPTION AND RISK OF THE TENDERING PREFERRED SHAREHOLDER. IF CERTIFICATES FOR
SHARES ARE SENT BY MAIL, REGISTERED MAIL WITH RETURN RECEIPT REQUESTED, PROPERLY
INSURED, IS RECOMMENDED.
 
     No alternative, conditional or contingent tenders will be accepted. See
"Terms of the Offer -- Number of Shares; Purchase Prices; Expiration Date;
Dividends" in the Offer to Purchase and Proxy Statement. By executing this
Letter of Transmittal and Proxy, the tendering shareholder waives any right to
receive any notice of the acceptance for payment of the Shares.
 
   
     3. VOTING. WHILE PREFERRED SHAREHOLDERS WHO WISH TO TENDER THEIR SHARES
PURSUANT TO THE OFFER NEED NOT VOTE IN FAVOR OF THE PROPOSED AMENDMENT, THE
OFFER IS CONDITIONED UPON THE PROPOSED AMENDMENT BEING ADOPTED AT THE SPECIAL
MEETING. In addition, Preferred Shareholders have the right to vote for the
Proposed Amendment regardless of whether they tender their Shares by casting
their vote and duly executing the Separate Proxy enclosed herewith or by voting
in person at the Special Meeting. By executing a Notice of
    
<PAGE>   9
 
Guaranteed Delivery, a Preferred Shareholder is deemed to have tendered the
Shares described in such Notice of Guaranteed Delivery and to have voted such
Shares in accordance with the proxy returned therewith, if any. If no vote is
indicated on an otherwise properly executed proxy, then all Shares in respect of
such proxy will be voted in favor of the Proposed Amendment. See "Proposed
Amendment and Proxy Solicitation" in the Offer to Purchase and Proxy Statement.
The Offer is being sent to all persons in whose names Shares are registered on
the books of IPC on the Record Date (as defined in the Offer to Purchase and
Proxy Statement) and transferees thereof. Only a record holder of Shares on the
Record Date may vote in person or by proxy at the Special Meeting. No record
date is fixed for determining which persons are permitted to tender Shares. Any
person who is the beneficial owner but not the record holder of Shares must
arrange for the record transfer of such Shares prior to tendering.
 
     4. PARTIAL TENDERS (NOT APPLICABLE TO SHAREHOLDERS WHO TENDER BY BOOK-ENTRY
TRANSFER). If fewer than all the Shares represented by any certificate delivered
to the Depositary are to be tendered, fill in the number of Shares that are to
be tendered in the box above under the heading "Description of Shares Tendered".
In such case, a new certificate for the remainder of the Shares represented by
the old certificate will be sent to the person(s) signing this Letter of
Transmittal and Proxy, unless otherwise provided in the box above under the
heading "Special Payment Instructions" or "Special Delivery Instructions", as
promptly as practicable following the expiration or termination of the Offer.
All Shares represented by certificates delivered to the Depositary will be
deemed to have been tendered unless otherwise indicated.
 
     5. SIGNATURES ON THIS LETTER OF TRANSMITTAL AND PROXY AND/OR NOTICE OF
GUARANTEED DELIVERY; STOCK POWERS AND ENDORSEMENTS. If either this Letter of
Transmittal and Proxy or the Notice of Guaranteed Delivery (together, the
"Tender Documents") is signed by the registered holder(s) of the Shares tendered
hereby, the signature(s) must correspond with the name(s) as written on the face
of the certificates without alteration, enlargement or any change whatsoever. If
any of the Shares tendered under either Tender Document is held of record by two
or more persons, all such persons must sign such Tender Document. If any of the
Shares tendered under either Tender Document is registered in different names or
different certificates, it will be necessary to complete, sign and submit as
many separate applicable Tender Documents as there are different registrations
or certificates. If either Tender Document is signed by the registered holder(s)
of the Shares tendered hereby, no endorsements of certificates or separate stock
powers are required unless payment of the purchase price is to be made to, or
Shares not tendered or not purchased are to be registered in the name of, any
person other than the registered holder(s). Signatures on any such certificates
or stock powers must be guaranteed by an Eligible Institution. See Instruction
1. If this Letter of Transmittal and Proxy is signed by a person other than the
registered holder(s) of the Shares tendered hereby, certificates must be
endorsed or accompanied by appropriate stock powers, in either case, signed
exactly as the name(s) of the registered holder(s) appear(s) on the certificates
for such Shares. Signature(s) on any such certificates or stock powers must be
guaranteed by an Eligible Institution. See Instruction 1. If either Tender
Document or any certificate or stock power is signed by a trustee, executor,
administrator, guardian, attorney-in-fact, officer of a corporation or other
person acting in a fiduciary or representative capacity, such person should so
indicate when signing, and proper evidence satisfactory to ILN of the authority
of such person so to act must be submitted.
 
     6. STOCK TRANSFER TAXES. Except as set forth in this Instruction 6, ILN
will pay or cause to be paid any stock transfer taxes with respect to the sale
and transfer of any Shares to it or its order pursuant to the Offer. If,
however, payment of the purchase price is to be made to, or Shares not tendered
or not purchased are to be registered in the name of, any person other than the
registered holder(s), or if tendered Shares are registered in the name of any
person other than the person(s) signing this Letter of Transmittal and Proxy,
the amount of any stock transfer taxes (whether imposed on the registered
holder(s), such other person or otherwise) payable on account of the transfer to
such person will be deducted from the purchase price unless satisfactory
evidence of the payment of such taxes, or exemption therefrom, is submitted. See
"Terms of the Offer -- Acceptance of Shares for Payment and Payment of Purchase
Price" in the Offer to Purchase and Proxy Statement. EXCEPT AS PROVIDED IN THIS
INSTRUCTION 6, IT WILL NOT BE NECESSARY TO AFFIX TRANSFER TAX STAMPS TO THE
CERTIFICATES REPRESENTING SHARES TENDERED HEREBY.
<PAGE>   10
 
     7. SPECIAL PAYMENT AND DELIVERY INSTRUCTIONS. If the check for the purchase
price of any Shares purchased is to be issued in the name of, and/or any Shares
not tendered or not purchased are to be returned to, a person other than the
person(s) signing this Letter of Transmittal and Proxy or if the check and/or
any certificate for Shares not tendered or not purchased are to be mailed to
someone other than the person(s) signing this Letter of Transmittal and Proxy or
to an address other than that shown in the box above under the heading "Name(s)
and Address(es) of Registered Holder(s)", then the "Special Payment
Instructions" and/or "Special Delivery Instructions" on this Letter of
Transmittal and Proxy should be completed. Preferred Shareholders tendering
Shares by book-entry transfer will have any Shares not accepted for payment
returned by crediting the account maintained by such Preferred Shareholder at
the Book-Entry Transfer Facility from which such transfer was made.
 
     8. SUBSTITUTE FORM W-9 AND FORM W-8. The tendering Preferred Shareholder is
required to provide the Depositary with either a correct Taxpayer Identification
Number ("TIN") on Substitute Form W-9, which is provided under "Important Tax
Information" below, or a properly completed Form W-8. Failure to provide the
information on either Substitute Form W-9 or Form W-8 may subject the tendering
Preferred Shareholder to a $50 penalty imposed by the Internal Revenue Service
and to 31% federal income tax backup withholding on the payment of the purchase
price for the Shares. The box in Part 2 of Substitute Form W-9 may be checked if
the tendering Preferred Shareholder has not been issued a TIN and has applied
for a number or intends to apply for a number in the near future. If the box in
Part 2 is checked and the Depositary is not provided with a TIN by the time of
payment, the Depositary will withhold 31% on all payments of the purchase price
for the Shares thereafter until a TIN is provided to the Depositary.
 
     9. REQUESTS FOR ASSISTANCE OR ADDITIONAL COPIES. Any questions or requests
for assistance may be directed to the Information Agent or the Dealer Manager at
their respective telephone numbers and addresses listed below. Requests for
additional copies of the Offer to Purchase and Proxy Statement, this Letter of
Transmittal and Proxy or other related tender offer materials may be directed to
the Information Agent and such copies will be furnished promptly at ILN's
expense. Preferred Shareholders may also contact their local broker, dealer,
commercial bank or trust company for assistance concerning the Offer.
 
     10. SOLICITED TENDERS. ILN will pay a solicitation fee of $     per Share
that is tendered, accepted for payment and paid for pursuant to the Offer in
transactions for beneficial owners of fewer than      Shares and a solicitation
fee of $     per Share for transactions for beneficial owners of      or more
Shares; provided that solicitation fees payable in transactions for beneficial
owners of      or more Shares shall be paid 80% to the Dealer Manager and 20% to
any designated Soliciting Dealer (which may be the Dealer Manager) covered by
the Letter of Transmittal and Proxy which designates, under the heading
"Solicited Tenders," as having solicited and obtained the tender, the name of
(a) any broker or dealer in securities, including a Dealer Manager in its
capacity as a dealer or broker, which is a member of any national securities
exchange or of the NASD, (b) any foreign broker or dealer not eligible for
membership in the NASD which agrees to conform to the NASD's Rules of Fair
Practice in soliciting tenders outside the United States to the same extent as
though it were an NASD member, or (c) any bank or trust company (each of which
is referred to herein as a "Soliciting Dealer"). No such fee shall be payable to
a Soliciting Dealer with respect to the tender of Shares by a holder unless the
Letter of Transmittal and Proxy accompanying such tender designates such
Soliciting Dealer. No such fee shall be payable to a Soliciting Dealer in
respect of Shares registered in the name of such Soliciting Dealer unless such
Shares are held by such Soliciting Dealer as nominee and such Shares are being
tendered for the benefit of one or more beneficial owners identified on the
Letter of Transmittal and Proxy or on the Notice of Solicited Tenders (included
in the materials provided to brokers and dealers). No such fee shall be payable
to a Soliciting Dealer with respect to the tender of Shares by the holder of
record, for the benefit of the beneficial owner, unless the beneficial owner has
designated such Soliciting Dealer. If tendered Shares are being delivered by
book-entry transfer, the Soliciting Dealer must return a Notice of Solicited
Tenders to the Depositary within three business days after expiration of the
Offer to receive a solicitation fee. No such fee shall be payable to a
Soliciting Dealer if such Soliciting Dealer is required for any reason to
transfer the amount of such fee to a depositing holder (other than itself). No
such fee shall be paid to a Soliciting Dealer with respect to Shares tendered
for such Soliciting Dealer's own account. No broker, dealer, bank, trust company
or fiduciary shall be deemed to be the agent of ILN, the Depositary, the
Information
<PAGE>   11
 
   
Agent or the Dealer Manager for purposes of the Offer. Soliciting Dealers will
include any organizations described in clauses (a), (b) or (c) above even when
the activities of such organization in connection with the Offer consist solely
of forwarding to clients materials relating to the Offer, including this Letter
of Transmittal and Proxy, and rendering Shares as directed by beneficial owners
thereof. No Soliciting Dealer is required to make any recommendation to holders
of Shares as to whether to tender or refrain from tendering in the Offer. No
assumption is made, in making payment to any Soliciting Dealer, that its
activities in connection with the Offer included any activities other than those
described above, and for all purposes noted in all materials relating to the
Offer, the term "solicit" shall be deemed to mean no more than "processing
shares tendered" or "forwarding to customers materials regarding the Offer".
    
 
     11. IRREGULARITIES. All questions as to the form of documents and the
validity, eligibility (including time of receipt) and acceptance of any tender
of Shares will be determined by ILN, in its sole discretion, and its
determination shall be final and binding. ILN reserves the absolute right to
reject any and all tenders of Shares that it determines are not in proper form
or the acceptance for payment of or payment for Shares that may, in the opinion
of ILN's counsel, be unlawful. ILN also reserves the absolute right to waive any
of the conditions to the Offer or any defect or irregularity in any tender of
Shares and ILN's interpretation of the terms and conditions of the Offer
(including these instructions) shall be final and binding. Unless waived, any
defects or irregularities in connection with tenders must be cured within such
time as ILN shall determine. None of ILN, IPC, the Dealer Manager, the
Depositary, the Information Agent or any other person shall be under any duty to
give notice of any defect or irregularity in tenders, nor shall any of them
incur any liability for failure to give any such notice. Tenders will not be
deemed to have been made until all defects and irregularities have been cured or
waived.
 
     12. LOST, DESTROYED OR STOLEN CERTIFICATES. If any of your certificate(s)
for Shares have been lost, stolen or destroyed, please call the ILN's
Shareholder Services Department at (800)800-8220. In addition, you should advise
ILN's Shareholder Services Department of any certificate(s) you have in your
possession. You will need to complete an Affidavit of Loss with respect to the
lost certificate(s) (which will be provided by ILN's Shareholder Services
Department) and pay an indemnity bond premium fee. The tender of Shares pursuant
to this Letter of Transmittal and Proxy will not be valid unless prior to the
Expiration Date: (a) such procedures have been completed and a replacement
certificate for the Shares has been delivered to the Depositary or (b) a Notice
of Guaranteed Delivery has been delivered to the Depositary. See Instruction 2.
 
     IMPORTANT: THIS LETTER OF TRANSMITTAL AND PROXY, DULY EXECUTED, TOGETHER
WITH, IF APPLICABLE, CERTIFICATES OR CONFIRMATION OF BOOK-ENTRY TRANSFER, AND
ALL OTHER REQUIRED DOCUMENTS MUST BE RECEIVED BY THE DEPOSITARY, OR, IF
APPLICABLE, THE NOTICE OF GUARANTEED DELIVERY MUST BE RECEIVED BY THE
DEPOSITARY, ON OR PRIOR TO THE EXPIRATION DATE.
 
                           IMPORTANT TAX INFORMATION
 
     Under United States Federal income tax law, a Preferred Shareholder whose
tendered Shares are accepted for payment is required to provide the Depositary
(as payer) with either such Preferred Shareholder's correct TIN on Substitute
Form W-9 or a properly completed Form W-8. If such Preferred Shareholder is an
individual, the TIN is his or her social security number. For businesses and
other entities, the number is the Federal employer identification number. If the
Depositary is not provided with the correct TIN or properly completed Form W-8,
the Preferred Shareholder may be subject to a $50 penalty imposed by the
Internal Revenue Code. In addition, payments that are made to such Preferred
Shareholder with respect to Shares purchased pursuant to the Offer may be
subject to 31% backup withholding. To avoid backup withholding, an individual
Preferred Shareholder is required to complete the Substitute Form W-9 attached
hereto certifying that the TIN provided on Substitute Form W-9 is correct and
that (a) the Preferred Shareholder has not been notified by the Internal Revenue
Service that he or she is subject to United States Federal income tax backup
withholding as a result of a failure to report all interest or dividends or (b)
the Internal Revenue Service has notified the Preferred Shareholder that he or
she is no longer subject to United
<PAGE>   12
 
States Federal income tax backup withholding. See the enclosed Guidelines for
Certification of Taxpayer Identification Number on Substitute Form W-9 for
additional instructions.
 
     Certain Preferred Shareholders (including, among others, all corporations
and certain foreign individuals) are exempt from backup withholding. For a
corporate United States Preferred Shareholder to qualify for such exemption,
such Preferred Shareholder must provide the Depositary with a properly completed
and executed Substitute Form W-9 attesting to its exempt status. In order for a
foreign Preferred Shareholder to qualify as an exempt recipient, such Preferred
Shareholder must submit to the Depositary a properly completed Internal Revenue
Service Form W-8, signed under penalties of perjury attesting to that Preferred
Shareholder's exempt status. A Form W-8 can be obtained from the Depositary.
 
     If United States Federal income tax backup withholding applies, the
Depositary is required to withhold 31% of any payments made to the Preferred
Shareholder. Backup withholding is not an additional tax. Rather, the United
States Federal income tax liability of persons subject to backup withholding
will be reduced by the amount of the tax withheld. If withholding results in an
overpayment of taxes, a refund may be obtained from the Internal Revenue
Service.
 
WHAT NUMBER TO GIVE THE DEPOSITARY
 
     The Preferred Shareholder is required to give the Depositary the social
security number or employer identification number of the registered owner of the
Shares. If the Shares are in more than one name or are not in the name of the
actual owner, consult the enclosed Guidelines for Certification of Taxpayer
Identification Number on Substitute Form W-9 for additional guidance on which
number to report.
<PAGE>   13
 
<TABLE>
<S>                          <C>                                                         <C>
- ------------------------------------------------------------------------------------------------------------------------
                                 PAYER'S NAME: FIRST CHICAGO TRUST COMPANY OF NEW YORK
- ------------------------------------------------------------------------------------------------------------------------
                               PART 1--PLEASE PROVIDE YOUR TIN IN THE BOX AT RIGHT AND
  SUBSTITUTE                   CERTIFY BY SIGNING AND DATING BELOW.                       -----------------------------
  FORM W-9                                                                                  Social Security Number(s)
                                                                                          OR --------------------------
                                                                                         Employer Identification Number
                             -------------------------------------------------------------------------------------------
 DEPARTMENT OF THE TREASURY    PART 2--                                                             PART 3--
 INTERNAL REVENUE SERVICE      CERTIFICATION -- Under Penalties of Perjury, I certify           Awaiting TIN  [ ]
                               that:
                               (1) The number shown on this form is my correct taxpayer
                                   identification number (or a TIN has not been issued
                                   to me but I have mailed or delivered an application
                                   to receive a TIN or intend to do so in the near
                                   future),
                               (2) I am not subject to backup withholding either
                               because (a) I have not been notified by the Internal
                                   Revenue Service (the "IRS") that I am subject to
                                   backup withholding as a result of a failure to
                                   report all interest or dividends, or (b) the IRS has
                                   notified me that I am no longer subject to backup
                                   withholding.
                             -------------------------------------------------------------------------------------------
                               You must cross out item (2) above if you have been notified by the IRS that you are
  PAYER'S REQUEST FOR          currently subject to backup withholding because of underreporting interest or dividends
  TAXPAYER IDENTIFICATION      on your tax return.
  NUMBER (TIN)                 SIGNATURE _________________________________  DATE  ____________________________  1998
- -----------------------------------------------------------------------------------------------------------------------
</TABLE>
 
NOTE: FAILURE TO COMPLETE AND RETURN THIS FORM MAY RESULT IN BACKUP WITHHOLDING
      OF 31% OF ANY PAYMENTS MADE TO YOU PURSUANT TO THE OFFER. PLEASE REVIEW
      THE ENCLOSED CERTIFICATION "GUIDELINES FOR CERTIFICATION OF TAXPAYER
      IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9" FOR ADDITIONAL DETAILS.
 
NOTE: YOU MUST COMPLETE THE FOLLOWING CERTIFICATE IF YOU CHECKED THE BOX IN PART
      2 OF SUBSTITUTE FORM W-9.
 
             CERTIFICATE OF AWAITING TAXPAYER IDENTIFICATION NUMBER
 
  I certify under penalties of perjury that a taxpayer identification number has
not been issued to me and either (1) I have mailed or delivered an application
to receive a taxpayer identification number to the appropriate Internal Revenue
Service Center or Social Security Administration Office or (2) I intend to do so
in the near future. I understand that if I do not provide a taxpayer
identification number by the time of payment, 31% of all payments of the
purchase price made to me will be withheld until I provide such a number.
 
SIGNATURE  __________________________________________________  DATE _______1998
<PAGE>   14
 
   The Information Agent for the Offer to Purchase and Proxy Solicitation is:
                            MACKENZIE PARTNERS, INC.
                                156 Fifth Avenue
                            New York, New York 10010
                           (800) 322-2885 (Toll Free)
                         (212) 929-5500 (Call Collect)
 
    The Dealer Manager for the Offer to Purchase and Proxy Solicitation is:
                          DONALDSON, LUFKIN & JENRETTE
                                   SECURITIES CORPORATION
                                277 Park Avenue
                            New York, New York 10172
                           (800) 334-1604 (Toll Free)
                         (212) 892-3351 (Call Collect)
                        Attn: Paul Galant or Jeff Dorst

<PAGE>   1
 
              TO BE COMPLETED ONLY BY THOSE PREFERRED SHAREHOLDERS
                WHO ARE NOT TENDERING THEIR SHARES IN THE OFFER
   
                         % SERIES CUMULATIVE PREFERRED STOCK
    
   
                           CUSIP NUMBER 452092
    
                                     PROXY
 
     The undersigned hereby appoints        ,        and        , or any of
them, as proxies, each with the power to appoint his substitute, and hereby
authorizes them to represent and to vote as designated hereunder and in their
discretion with respect to any other business properly brought before the
Special Meeting, all the shares of cumulative preferred stock of Illinois Power
Company ("IPC") which the undersigned is entitled to vote at the Special Meeting
of Shareholders to be held on                , 1998, or any adjournment(s) or
postponement(s) thereof.
 
     THIS PROXY IS SOLICITED ON BEHALF OF THE BOARD OF DIRECTORS OF IPC. When
properly executed, this Proxy will be voted in the manner directed herein by the
undersigned shareholder(s). If no direction is made, the Proxy will be voted for
Item 1.
 
     INDICATE YOUR VOTE BY AN (X). THE BOARD OF DIRECTORS OF IPC RECOMMENDS
VOTING FOR ITEM 1.
 
ITEM 1.
 
     To remove in its entirety ARTICLE V, Section 1, Clause (f)(1) from the
Amended and Restated Articles of Incorporation of IPC (the "Articles"), which
limits IPC's ability to issue or assume unsecured indebtedness.
 
       [ ] FOR                   [ ] AGAINST                  [ ] ABSTAIN
 
   
NOTE: THE SUBSTITUTE FORM W-9 BELOW SHOULD BE COMPLETED TO AVOID BACK-UP
WITHHOLDING ON THE SPECIAL CASH PAYMENT.
    
 
     SHARES REPRESENTED BY ALL PROPERLY EXECUTED PROXIES WILL BE VOTED IN
ACCORDANCE WITH INSTRUCTIONS APPEARING ON THIS PROXY. IN THE ABSENCE OF SPECIFIC
INSTRUCTIONS, PROXIES WILL BE VOTED IN ACCORDANCE WITH THE RECOMMENDATIONS OF
THE BOARD OF DIRECTORS OF IPC AND IN THE DISCRETION OF THE PROXY HOLDERS AS TO
ANY OTHER MATTERS THAT MAY PROPERLY COME BEFORE THE SPECIAL MEETING.
 
     Any holder of Shares held of record on the Record Date in the name of
another holder must establish to the satisfaction of IPC his or her entitlement
to exercise or transfer this Proxy. This will ordinarily require an assignment
by such record holder in blank, or if not in blank, to and from each successive
transferee, including the holder, with each signature guaranteed by an Eligible
Institution.
 
Please check box if you plan to attend the Special Meeting. [ ]
 
                       Signature
 
                      ------------------------------------------------     Date:
                      -----------------------------------------------, 1998
 
                       Print Name:
                       ---------------------------------------------
<PAGE>   2
 
                           NOTICE OF SOLICITED VOTES
 
     If the Proposed Amendment is adopted at the Special Meeting, IPC will pay
to each designated Soliciting Dealer a separate fee of $     per Share for
Shares that are not tendered pursuant to the Offer but which are voted in favor
of the Proposed Amendment in transactions for beneficial owners of fewer than
         Shares and a separate fee of $     per Share for Shares that are not
tendered pursuant to the Offer but which are voted in favor of the Proposed
Amendment in transactions for beneficial owners of      or more Shares;
provided that the separate fee payable in transactions for beneficial owners of
or more Shares shall be paid 80% to the Dealer Manager and 20% to any
designated Soliciting Dealer (which may be the Dealer Manager). However,
Soliciting Dealers will not be entitled to a fee for Shares beneficially owned
by such Soliciting Dealer. For purposes of this paragraph, a "Soliciting
Dealer" shall include: (a) any broker or dealer in securities, including a
Dealer Manager in its capacity as a dealer or broker, which is a member of any
national securities exchange or of the NASD, (b) any foreign broker or dealer
not eligible for membership in the NASD which agrees to conform to the NASD's
Rules of Fair Practice in soliciting votes outside the United States to the
same extent as though it were an NASD member, or (c) any bank or trust company.
No fee shall be payable to a Soliciting Dealer with respect to the vote of
Shares in favor of the Proposed Amendment by a holder unless the Proxy relating
to such Shares designates such Soliciting Dealer. No such fee shall be payable
to a Soliciting Dealer in respect of Shares registered in the name of such
Soliciting Dealer unless such Shares are held by such Soliciting Dealer as
nominee and such Shares are being voted for the benefit of one or more
beneficial owners identified on the Proxy. No such fee shall be payable to a
Soliciting Dealer with respect to the vote of Shares by the holder of record,
for the benefit of the beneficial owner, unless the beneficial owner has
designated such Soliciting Dealer in the Letter from the Dealer Manager to
Brokers, Dealers, Commercial Banks, Trust Companies and Other Nominees (the
"Broker Letter").
        
     The above signed represents that the Soliciting Dealer which solicited and
obtained this vote in favor of the Proposed Amendment is:
 
Name of Firm:
- --------------------------------------------------------------------------------
                                 (Please Print)
 
Name of Individual Broker
or Financial Consultant:
- --------------------------------------------------------------------------------
                                 (Please Print)
 
Telephone Number of Broker
or Financial Consultant:
- --------------------------------------------------------------------------------
 
Identification Number (if known):
- ---------------------------------------------------------------------------
 
Address:
- --------------------------------------------------------------------------------
                               (Include Zip Code)
 
     The following to be completed ONLY if customer's Shares held in nominee
name are tendered.
 
                            NAME OF BENEFICIAL OWNER
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
- ------------------------------------------------------
                         NUMBER OF SHARES VOTED FOR THE
                               PROPOSED AMENDMENT
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
                     (ATTACH ADDITIONAL LIST IF NECESSARY)
 
     The acceptance of compensation by such Soliciting Dealer will constitute a
representation by it that (a) it has complied with the applicable requirements
of the Securities Exchange Act of 1934, as amended, and the applicable rules and
regulations thereunder, in connection with such solicitation; (b) it is entitled
to such compensation for such solicitation under the terms and conditions of the
Offer to Purchase and Proxy
<PAGE>   3
 
Statement; (c) in soliciting votes in favor of the Proposed Amendment, it has
used no solicitation materials other than those furnished by IPC; and (d) if it
is a foreign broker or dealer not eligible for membership in the National
Association of Securities Dealers, Inc. (the "NASD"), it has agreed to conform
to the NASD's Rules of Fair Practice in making solicitations.
 
     The payment of compensation to any Soliciting Dealer is dependent on such
Soliciting Dealer returning a Notice of Solicited Votes to the Depositary.
<PAGE>   4
 
<TABLE>
<S>                          <C>                                                         <C>
- ----------------------------------------------------------------------------------------------------------------------------
                                 PAYER'S NAME: FIRST CHICAGO TRUST COMPANY OF NEW YORK
- ----------------------------------------------------------------------------------------------------------------------------
                               PART 1--PLEASE PROVIDE YOUR TIN IN THE BOX AT RIGHT AND
  SUBSTITUTE                   CERTIFY BY SIGNING AND DATING BELOW.                       ----------------------------------
  FORM W-9                                                                                   Social Security Number(s)

                                                                                          OR -------------------------------
                                                                                             Employer Identification Number
                             -----------------------------------------------------------------------------------------------
 DEPARTMENT OF THE TREASURY    PART 2--                                                   PART 3--
 INTERNAL REVENUE SERVICE      CERTIFICATION -- Under Penalties of Perjury, I certify     Awaiting TIN  [ ]
                               that:
                               (1) The number shown on this form is my correct taxpayer
                                   identification number (or a TIN has not been issued
                                   to me but I have mailed or delivered an application
                                   to receive a TIN or intend to do so in the near
                                   future),
                               (2) I am not subject to backup withholding either
                               because (a) I have not been notified by the Internal
                                   Revenue Service (the "IRS") that I am subject to
                                   backup withholding as a result of a failure to
                                   report all interest or dividends, or (b) the IRS has
                                   notified me that I am no longer subject to backup
                                   withholding.
                             ------------------------------------------------------------------------------------------------
                               You must cross out item (2) above if you have been notified by the IRS that you are
  PAYER'S REQUEST FOR          currently subject to backup withholding because of underreporting interest or dividends
  TAXPAYER IDENTIFICATION      on your tax return.
  NUMBER (TIN)                 SIGNATURE _________________________________  DATE  ____________________________  1998
- ----------------------------------------------------------------------------------------------------------------------------
</TABLE>
 

NOTE: FAILURE TO COMPLETE AND RETURN THIS FORM MAY RESULT IN BACKUP WITHHOLDING
      OF 31% OF ANY SPECIAL CASH PAYMENTS MADE TO YOU. PLEASE REVIEW THE
      ENCLOSED CERTIFICATION "GUIDELINES FOR CERTIFICATION OF TAXPAYER
      IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9" FOR ADDITIONAL DETAILS.
 
NOTE: YOU MUST COMPLETE THE FOLLOWING CERTIFICATE IF YOU CHECKED THE BOX IN PART
      2 OF SUBSTITUTE FORM W-9.
 
             CERTIFICATE OF AWAITING TAXPAYER IDENTIFICATION NUMBER
 
  I certify under penalties of perjury that a taxpayer identification number has
not been issued to me and either (1) I have mailed or delivered an application
to receive a taxpayer identification number to the appropriate Internal Revenue
Service Center or Social Security Administration Office or (2) I intend to do so
in the near future. I understand that if I do not provide a taxpayer
identification number by the time of payment, 31% of all payments of the
purchase price made to me will be withheld until I provide such a number.
 
SIGNATURE  __________________________________________________  DATE  1998


<PAGE>   1
 
                         NOTICE OF GUARANTEED DELIVERY
                                      FOR
 
                              ILLINOVA CORPORATION
                           OFFER TO PURCHASE FOR CASH
                         ANY AND ALL OUTSTANDING SHARES
            OF THE FOLLOWING SERIES OF CUMULATIVE PREFERRED STOCK OF
 
                             ILLINOIS POWER COMPANY
                  CUMULATIVE PREFERRED STOCK ($50 PAR VALUE):
                                  4.08% SERIES
                                  4.20% SERIES
                                  4.26% SERIES
                                  4.42% SERIES
                                  4.70% SERIES
                                  7.75% SERIES
 
   
     This form, or a form substantially equivalent to this form, must be used to
accept the Offer (as defined below) if certificates for shares of a series of
cumulative preferred stock (each a "Series of Preferred") of Illinois Power
Company ("IPC"), an Illinois corporation and direct utility subsidiary of
Illinova Corporation, an Illinois corporation ("ILN"), to be tendered pursuant
to the Offer (the "Shares") are not immediately available, if the procedure for
book-entry transfer cannot be completed on a timely basis, or if time will not
permit all other documents required by the Letter of Transmittal and Proxy (as
defined in the Offer to Purchase and Proxy Statement, which is defined below) to
be delivered to the Depositary (as defined below) on or prior to the Expiration
Date (as defined in the Offer to Purchase and Proxy Statement). Such form may be
delivered by hand or transmitted by mail to the Depositary. See "Terms of the
Offer -- Procedure for Tendering Shares -- Guaranteed Delivery Procedure" in the
Offer to Purchase and Proxy Statement.
    
 
   
     A SEPARATE NOTICE OF GUARANTEED DELIVERY MUST BE USED FOR EACH SERIES OF
PREFERRED.
    
 
   
     THE ELIGIBLE INSTITUTION (AS DEFINED IN THE LETTER OF TRANSMITTAL AND
PROXY) WHICH COMPLETES THIS FORM MUST DELIVER THE LETTER OF TRANSMITTAL AND
PROXY AND CERTIFICATES FOR SHARES TO THE DEPOSITARY WITHIN THE TIME SHOWN
HEREIN. FAILURE TO DO SO COULD RESULT IN A FINANCIAL LOSS TO SUCH ELIGIBLE
INSTITUTION.
    
 
   To: First Chicago Trust Company of New York, Depositary (the "Depositary")
 
<TABLE>
<CAPTION>
           By Mail:                         By Hand:                   By Overnight Courier:
           --------                         --------                   ---------------------
<S>                              <C>                              <C>
First Chicago Trust Company of   First Chicago Trust Company of   First Chicago Trust Company of
           New York                         New York                         New York
   Attn: Tenders & Exchanges        Attn: Tenders & Exchanges        Attn: Tenders & Exchanges
         P.O. Box 2565              c/o The Depository Trust              Suite 4680-CBE
          Suite 4660                         Company                      14 Wall Street
  Jersey City, NJ 07303-2565        55 Water Street, DTC TAD                 8th Floor
                                 Vietnam Veterans Memorial Plaza        New York, NY 10005
                                       New York, NY 10041
</TABLE>
 
  By Facsimile: (201) 222-4720 or 222-4721         Confirm by Telephone: (201)
                                    222-4707
 
     DELIVERY OF THIS INSTRUMENT TO AN ADDRESS OTHER THAN AS SET FORTH ABOVE
WILL NOT CONSTITUTE A VALID DELIVERY.
<PAGE>   2
 
     This form is not to be used to guarantee signatures. If a signature on a
Letter of Transmittal and Proxy is required to be guaranteed by an Eligible
Institution under the instructions thereto, such signature guarantee must appear
in the applicable space provided in the signature box on the Letter of
Transmittal and Proxy.
 
     The undersigned hereby tenders to ILN, upon the terms and subject to the
conditions set forth in the Offer to Purchase and Proxy Statement, dated
                    (the "Offer to Purchase and Proxy Statement"), and the
related Letter of Transmittal and Proxy (which together constitute the "Offer"),
receipt of which is hereby acknowledged, the number of Shares listed below,
pursuant to the guaranteed delivery procedure set forth in "Terms of the Offer
- -- Procedure for Tendering Shares" in the Offer to Purchase and Proxy Statement.
 
   
     WHILE PREFERRED SHAREHOLDERS WHO WISH TO TENDER THEIR SHARES PURSUANT TO
THE OFFER NEED NOT VOTE IN FAVOR OF THE PROPOSED AMENDMENT TO IPC'S AMENDED AND
RESTATED ARTICLES OF INCORPORATION, AS SET FORTH IN THE OFFER TO PURCHASE AND
PROXY STATEMENT (THE "PROPOSED AMENDMENT"), THE OFFER IS CONDITIONED UPON THE
PROPOSED AMENDMENT BEING ADOPTED AT THE SPECIAL MEETING (AS DEFINED IN THE OFFER
TO PURCHASE AND PROXY STATEMENT). IN ADDITION, PREFERRED SHAREHOLDERS OF RECORD
HAVE THE RIGHT TO VOTE FOR THE PROPOSED AMENDMENT REGARDLESS OF WHETHER THEY
TENDER THEIR SHARES BY CASTING THEIR VOTE AND SIGNING THE SEPARATE PROXY
ENCLOSED HEREWITH (OR THE PROXY CONTAINED IN THE LETTER OF TRANSMITTAL AND
PROXY) OR BY VOTING IN PERSON AT THE SPECIAL MEETING. IF THE PROPOSED AMENDMENT
IS ADOPTED BY IPC'S SHAREHOLDERS, IPC WILL MAKE A SPECIAL CASH PAYMENT (AS
DEFINED IN THE OFFER TO PURCHASE AND PROXY STATEMENT) TO EACH PREFERRED
SHAREHOLDER OF RECORD AS OF [RECORD DATE], PROVIDED THAT SUCH PREFERRED
SHAREHOLDER'S SHARES ARE NOT TENDERED PURSUANT TO THE OFFER.
    
<PAGE>   3

<TABLE>
<S>                                                                     <C>      
 Series of Preferred                                                    Signature(s) of Holder(s):                            
 (Select One)                                                                                                                 
 [ ] 4.08% Series                                                       ----------------------------------------------------- 
 [ ] 4.20% Series                                                                                                             
 [ ] 4.26% Series                                                       ----------------------------------------------------- 
 [ ] 4.42% Series                                                                                                             
 [ ] 4.70% Series                                                       Name(s) of Holder(s):                                 
 [ ] 7.75% Series                                                                                                             
                                                                        ----------------------------------------------------- 
 Number of Shares:                                                                                                            
 --------------------------------                                       ----------------------------------------------------- 
                                                                                           (Please Type or Print)    
 Certificate Nos. (if available):                                                                                             
                                                                        Address and ZIP Code:                                 
 -----------------------------------------------------                                                                        
                                                                        ----------------------------------------------------- 
 -----------------------------------------------------                                                                        
                                                                        ----------------------------------------------------- 
 Check box if Shares will be delivered by                                                                                     
 book-entry transfer:                                                   Area Code and Telephone No.:                          
 [ ] The Depository Trust Company                                                                                             
                                                                        ----------------------------------------------------- 
                                                                                                                              
                                                                        Dated:                                                
                                                                              -----------------------------------------------
</TABLE>
 
                                   GUARANTEE
                    (NOT TO BE USED FOR SIGNATURE GUARANTEE)
 
      The undersigned, a firm that is a member of a registered national
 securities exchange or the National Association of Securities Dealers, Inc. or
 a commercial bank or trust company having an office or correspondent in the
 United States, guarantees (a) that the above-named person(s) has a net long
 position in the Shares being tendered within the meaning of Rule 14e-4
 promulgated under the Securities Exchange Act of 1934, as amended, (b) that
 such tender of Shares complies with such Rule 14e-4 and (c) to deliver to the
 Depositary at one of its addresses set forth above certificate(s) for the
 Shares tendered hereby, in proper form for transfer, or a confirmation of the
 book-entry transfer of the Shares tendered hereby into the Depositary's
 account at The Depository Trust Company together with a properly completed and
 duly executed Letter(s) of Transmittal and Proxy (or facsimile(s) thereof),
 with any required signature guarantee(s) and any other required documents, all
 within three New York Stock Exchange trading days after the date hereof.



<TABLE>
<S>                                                                     <C>
 
 -----------------------------------------------------                  -----------------------------------------------------
 Name of Firm                                                           Authorized Signature                                 
                                                                                                                             
 -----------------------------------------------------                  -----------------------------------------------------
 Address                                                                Name                                                 
                                                                                                                             
 -----------------------------------------------------                  -----------------------------------------------------
 City, State, Zip Code                                                  Title                                                
                                                                                                                             
 -----------------------------------------------------                  Dated:                                         , 1998
 Area Code and Telephone Number                                               -----------------------------------------

</TABLE>
 
DO NOT SEND STOCK CERTIFICATES WITH THIS FORM. YOUR STOCK CERTIFICATES MUST BE
SENT WITH THE APPLICABLE LETTER OF TRANSMITTAL AND PROXY.

<PAGE>   1
 
                         [ILLINOIS POWER COMPANY LOGO]
 
                             ILLINOIS POWER COMPANY
                             500 SOUTH 27TH STREET
                            DECATUR, ILLINOIS 62525
 
                   NOTICE OF SPECIAL MEETING OF SHAREHOLDERS
          TO BE HELD                     ,                     , 1998
 
TO SHAREHOLDERS OF
ILLINOIS POWER COMPANY
 
     NOTICE IS HEREBY GIVEN that a Special Meeting of Shareholders of Illinois
Power Company ("IPC") will be held at its principal office, 500 South 27th
Street, Decatur, Illinois 62525 on                     ,                     ,
1998 at 4:30 p.m., New York City time, for the following purposes:
 
   
        1. To adopt an amendment to Illinois Power Company's Amended and
           Restated Articles of Incorporation to remove the limitation on the
           issuance or assumption of unsecured indebtedness; and
    
 
        2. To transact such other business as may properly come before the
           meeting or any adjournment thereof.
 
     The Board of Directors of IPC has fixed the close of business on
                    , 1998 as the record date for determining the shareholders
entitled to notice of, and to vote at, the meeting or any adjournment thereof.
 
   
     IT IS IMPORTANT THAT YOUR SHARES BE REPRESENTED AND VOTED AT THIS MEETING.
WHETHER OR NOT YOU EXPECT TO BE PRESENT AT THE MEETING, YOU ARE URGED TO SIGN
AND DATE THE ENCLOSED LETTER OF TRANSMITTAL AND PROXY OR SEPARATE PROXY, AS
APPROPRIATE, AND RETURN IT IMMEDIATELY TO THE DEPOSITARY IN THE ACCOMPANYING
POSTAGE-PAID ENVELOPE. SEE THE OFFER TO PURCHASE AND PROXY STATEMENT ENCLOSED
HEREWITH FOR FURTHER INSTRUCTIONS.
    
 
     By order of the Board of Directors,
 
                                          ILLINOIS POWER COMPANY
 
                                          --------------------------------------
                                          By: Leah Manning Stetzner,
                                            General Counsel and Corporate
                                              Secretary

<PAGE>   1
 
   
                          DONALDSON, LUFKIN & JENRETTE
    
   
      SECURITIES CORPORATION>
    
 
                             THE DEALER MANAGER FOR
 
                              ILLINOVA CORPORATION
                           OFFER TO PURCHASE FOR CASH
           ANY AND ALL OUTSTANDING SHARES OF THE FOLLOWING SERIES OF
                         CUMULATIVE PREFERRED STOCK OF
 
                             ILLINOIS POWER COMPANY
 
283,290 SHARES, CUMULATIVE PREFERRED STOCK, 4.08% SERIES AT A PURCHASE PRICE OF
$  PER SHARE, CUSIP NUMBER 452092 20 8
167,720 SHARES, CUMULATIVE PREFERRED STOCK, 4.20% SERIES AT A PURCHASE PRICE OF
$  PER SHARE, CUSIP NUMBER 452092 30 7
136,000 SHARES, CUMULATIVE PREFERRED STOCK, 4.26% SERIES AT A PURCHASE PRICE OF
$  PER SHARE, CUSIP NUMBER 452092 40 6
134,400 SHARES, CUMULATIVE PREFERRED STOCK, 4.42% SERIES AT A PURCHASE PRICE OF
$  PER SHARE, CUSIP NUMBER 452092 50 5
176,000 SHARES, CUMULATIVE PREFERRED STOCK, 4.70% SERIES AT A PURCHASE PRICE OF
$  PER SHARE, CUSIP NUMBER 452092 60 4
241,700 SHARES, CUMULATIVE PREFERRED STOCK, 7.75% SERIES AT A PURCHASE PRICE OF
$  PER SHARE, CUSIP NUMBER 452092 79 4
 
THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 5:00 P.M., NEW YORK CITY TIME, ON
            , UNLESS EXTENDED.
 
                                                                [STATEMENT DATE]
 
To Brokers, Dealers, Commercial Banks,
  Trust Companies and Other Nominees:
 
   
     We have been appointed by Illinova Corporation, an Illinois corporation
("ILN"), to act as Dealer Manager and in connection therewith are enclosing the
material listed below relating to the invitation of ILN to the holders of each
series of cumulative preferred stock of Illinois Power Company, an Illinois
corporation and direct utility subsidiary of ILN ("IPC"), listed above (each a
"Series of Preferred") to tender any and all of their shares of a Series of
Preferred ("Shares") for purchase at the purchase price per Share listed above
plus accrued and unpaid dividends for the Shares tendered, net to the seller in
cash, upon the terms and subject to the conditions set forth in the Offer to
Purchase and Proxy Statement, dated [STATEMENT DATE] (the "Offer to Purchase and
Proxy Statement"), in the Separate Proxy and in the Letter of Transmittal and
Proxy for the Shares tendered. As to each Series of Preferred, the Offer to
Purchase and Proxy Statement, together with the applicable Letter of Transmittal
and Proxy, constitutes the "Offer." ILN will purchase all Shares validly
tendered and not withdrawn, upon the terms and subject to the conditions of the
Offer. The Offer for a Series of Preferred is not conditioned upon any minimum
number of Shares of such Series of Preferred being tendered and is independent
of the Offer for any other Series of Preferred.
    
 
     WHILE PREFERRED SHAREHOLDERS WHO WISH TO TENDER THEIR SHARES PURSUANT TO
THE OFFER NEED NOT VOTE IN FAVOR OF THE PROPOSED AMENDMENT TO IPC'S AMENDED AND
RESTATED ARTICLES OF INCORPORATION, AS SET FORTH IN THE OFFER TO PURCHASE AND
PROXY STATEMENT (THE "PROPOSED AMENDMENT"), THE OFFER IS CONDITIONED UPON THE
PROPOSED AMENDMENT BEING ADOPTED AT THE SPECIAL MEETING (AS DEFINED IN THE OFFER
TO PURCHASE AND PROXY STATEMENT). IN ADDITION, PREFERRED SHAREHOLDERS OF RECORD
HAVE THE RIGHT TO VOTE FOR THE PROPOSED AMENDMENT REGARDLESS OF WHETHER THEY
TENDER
<PAGE>   2
 
   
THEIR SHARES BY CASTING THEIR VOTE AND SIGNING THE SEPARATE PROXY
ENCLOSED HEREWITH, BY CASTING THEIR VOTE AND SIGNING THE PROXY CONTAINED IN THE
LETTER OF TRANSMITTAL AND PROXY OR BY VOTING IN PERSON AT THE SPECIAL MEETING.
IF THE PROPOSED AMENDMENT IS ADOPTED BY IPC'S SHAREHOLDERS, IPC WILL MAKE A
SPECIAL CASH PAYMENT IN THE AMOUNT OF $     PER SHARE TO EACH PREFERRED
SHAREHOLDER OF RECORD AS OF [RECORD DATE], PROVIDED THAT SUCH SHARES HAVE NOT
BEEN TENDERED PURSUANT TO THE OFFER. SEE "PROPOSED AMENDMENT AND PROXY
SOLICITATION", "TERMS OF THE OFFER -- CERTAIN CONDITIONS OF THE OFFER" AND
"TERMS OF THE OFFER -- EXTENSION OF TENDER PERIOD; TERMINATION; AMENDMENTS" IN
THE OFFER TO PURCHASE AND PROXY STATEMENT.
    
 
     We are asking you to contact your clients for whom you hold Shares
registered in your name (or in the name of your nominee) or who hold Shares
registered in their own names. Please bring the Offer to their attention as
promptly as possible.
 
   
     ILN will pay a solicitation fee of $     per Share that is tendered,
accepted for payment and paid for pursuant to the Offer in transactions for
beneficial owners of fewer than      Shares and a solicitation fee of $     per
Share for transactions for beneficial owners of      or more Shares; provided
that solicitation fees payable in transactions for beneficial owners of      or
more Shares shall be paid 80% to the Dealer Manager and 20% to any designated
Soliciting Dealer (which may be the Dealer Manager). If the Proposed Amendment
is adopted at the Special Meeting, IPC will pay to each designated Soliciting
Dealer a separate fee of $     per Share for Shares that are not tendered
pursuant to the Offer but which are voted in favor of the Proposed Amendment in
transactions for beneficial owners of fewer than      Shares and a separate fee
of $     per Share for Shares that are not tendered pursuant to the Offer but
which are voted in favor of the Proposed Amendment in transactions for
beneficial owners of      or more Shares; provided that the separate fee payable
in transactions for beneficial owners of      or more Shares shall be paid 80%
to the Dealer Manager and 20% to any designated Soliciting Dealer (which may be
the Dealer Manager). A designated "Soliciting Dealer" is an entity obtaining the
tender or proxy, if the Letter of Transmittal and Proxy or the Separate Proxy,
as the case may be, shall include its name and it is (i) any broker or dealer in
securities, including the Dealer Manager in their capacity as a broker or
dealer, which is a member in good standing of any national securities exchange
or of the National Association of Securities Dealers, Inc. (the "NASD"), (ii)
any foreign broker or dealer not eligible for membership in the NASD which
agrees to conform to the NASD's Rules of Fair Practice in soliciting tenders
outside the United States to the same extent as though it were an NASD member,
or (iii) any bank or trust company. No solicitation fee or separate fee (other
than solicitation fees payable to the Dealer Manager as provided above) shall be
payable to a Soliciting Dealer with respect to the tender of Shares or the vote
of Shares by a holder unless the Letter of Transmittal and Proxy or the Separate
Proxy accompanying such tender or vote, as the case may be, designates such
Soliciting Dealer. No solicitation fee or separate fee shall be payable to a
Soliciting Dealer in respect of Shares registered in the name of such Soliciting
Dealer unless such Shares are held by such Soliciting Dealer as nominee and such
Shares are being tendered or voted for the benefit of one or more beneficial
owners identified on the Letter of Transmittal and Proxy or on the Notice of
Solicited Tenders (included below). No solicitation fee or separate fee shall be
payable to a Soliciting Dealer if such Soliciting Dealer is required for any
reason to transfer the amount of such fee to a depositing holder (other than
itself). No solicitation fee shall be paid to a Soliciting Dealer with respect
to Shares tendered for such Soliciting Dealer's own account and no separate fee
shall be paid to a Soliciting Dealer with respect to Shares voted for such
Soliciting Dealer's own account. A Soliciting Dealer shall not be entitled to a
solicitation fee or a separate fee for Shares beneficially owned by such
Soliciting Dealer. No broker, dealer, bank, trust company or fiduciary shall be
deemed to be the agent of ILN, IPC, the Depositary (as defined below), the
Dealer Manager or the Information Agent (as defined below) for purposes of the
Offer.
    
 
     Soliciting Dealers will include any of the organizations described in
clauses (i), (ii) and (iii) above even when the activities of such organizations
in connection with the Offer consist solely of forwarding to clients materials
relating to the Offer, including the Letter of Transmittal and Proxy and
tendering Shares as directed
<PAGE>   3
 
by beneficial owners thereof. No Soliciting Dealer is required to make any
recommendation to holders of Shares as to whether to tender or refrain from
tendering in the Offer. No assumption is made, in making payment to any
Soliciting Dealer, that its activities in connection with the Offer included any
activities other than those described above, and for all purposes noted in all
materials relating to the Offer, the term "solicit" shall be deemed to mean no
more than "processing shares tendered" or "forwarding to customers materials
regarding the Offer."
 
     ILN will also, upon request, reimburse Soliciting Dealers for reasonable
and customary handling and mailing expenses incurred by them in forwarding
materials relating to the Offer to their customers. ILN will pay all stock
transfer taxes applicable to its purchase of Shares pursuant to the Offer,
subject to Instruction 6 of the Letter of Transmittal and Proxy.
 
   
     IN ORDER FOR A SOLICITING DEALER TO RECEIVE A SOLICITATION FEE OR A
SEPARATE FEE, FIRST CHICAGO TRUST COMPANY OF NEW YORK, AS DEPOSITARY (THE
"DEPOSITARY"), MUST HAVE RECEIVED FROM SUCH SOLICITING DEALER A PROPERLY
COMPLETED AND DULY EXECUTED NOTICE OF SOLICITED TENDERS AND PROXIES IN THE FORM
ATTACHED HERETO WITHIN THREE BUSINESS DAYS AFTER THE EXPIRATION OF THE OFFER.
    
 
     For your information and for forwarding to your clients for whom you hold
Shares registered in your name (or in the name of your nominee), we are
enclosing the following documents:
 
     1. The Offer to Purchase and Proxy Statement, dated [STATEMENT DATE].
 
     2. A separate Letter of Transmittal and Proxy for each Series of Preferred
for your use and for the information of your clients.
 
   
     3. A Separate Proxy for each Series of Preferred for your use and for the
information of your clients.
    
 
     4. A letter to shareholders of IPC from its Chairman of the Board and Chief
Executive Officer.
 
     5. A Notice of Guaranteed Delivery to be used to accept the Offer if the
Shares and all other required documents cannot be delivered to the Depositary by
the applicable Expiration Date (as defined in the Offer to Purchase and Proxy
Statement).
 
     6. A form of letter which may be sent to your clients for whose accounts
you hold Shares registered in your name or in the name of your nominee, with
space for obtaining such clients' instructions with regard to the Offer and with
regard to the Proxy solicitation by IPC.
 
     7. Guidelines of the Internal Revenue Service for Certification of Taxpayer
Identification Number on Substitute Form W-9, providing information relating to
backup federal income tax withholding.
 
     8. A return envelope addressed to First Chicago Trust Company of New York,
the Depositary.
 
     9. A Notice of Special Meeting of Shareholders.
 
     Each Series of Preferred has its own Letter of Transmittal and Proxy, and
only the applicable Letter of Transmittal and Proxy for a particular Series or a
Notice of Guaranteed Delivery may be used to tender Shares of such Series of
Preferred.
 
     We urge you to contact your clients as promptly as possible. Please note
that the Offer and withdrawal rights will expire at 5:00 p.m., New York City
time, on [EXPIRATION DATE], unless the Offer is extended.
 
     NEITHER ILN, IPC, THEIR RESPECTIVE BOARDS OF DIRECTORS NOR ANY OF THEIR
RESPECTIVE OFFICERS MAKES ANY RECOMMENDATION TO ANY PREFERRED SHAREHOLDER AS TO
WHETHER TO TENDER ANY OR ALL SHARES. EACH PREFERRED SHAREHOLDER MUST MAKE HIS OR
HER OWN DECISION AS TO WHETHER TO TENDER SHARES AND, IF SO, HOW MANY SHARES TO
TENDER.
 
     Any questions or requests for assistance or additional copies of the
enclosed materials may be directed to MacKenzie Partners, Inc., the Information
Agent, or to us, as Dealer Manager, at the respective addresses and telephone
numbers set forth on the back cover of the enclosed Offer to Purchase and Proxy
Statement.
<PAGE>   4
 
Very truly yours,
 
DONALDSON, LUFKIN & JENRETTE
          SECURITIES
         CORPORATION
 
     NOTHING CONTAINED HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL CONSTITUTE YOU
OR ANY PERSON AS THE AGENT OF ILN, IPC, THE DEALER MANAGER, THE INFORMATION
AGENT OR THE DEPOSITARY, OR AUTHORIZE YOU OR ANY OTHER PERSON TO USE ANY
DOCUMENT OR MAKE ANY STATEMENT ON BEHALF OF ANY OF THEM IN CONNECTION WITH THE
OFFER OTHER THAN THE DOCUMENTS ENCLOSED HEREWITH AND THE STATEMENTS CONTAINED
THEREIN.
 
                    NOTICE OF SOLICITED TENDERS AND PROXIES
                   FOR THE CUMULATIVE PREFERRED STOCK OF IPC
 
     List below the number of Shares tendered or voted in favor of the Proposed
Amendment but not tendered by each beneficial owner whose tender or vote you
have solicited. All Shares in a Series of Preferred beneficially owned by a
beneficial owner, whether in one account or several, and in however many
capacities, must be aggregated for purposes of completing the table below. Any
questions as to what constitutes beneficial ownership should be directed to the
Depositary. If the space below is inadequate, list the Shares in a separate
signed schedule and affix the list to this Notice of Solicited Tenders and
Proxies.
 
     PLEASE DO NOT COMPLETE THE SECTIONS OF THE TABLE HEADED "TO BE COMPLETED
ONLY BY DEPOSITARY."
 
     ALL NOTICES OF SOLICITED TENDERS SHOULD BE RETURNED TO, AND ALL QUESTIONS
CONCERNING THE NOTICES OF SOLICITED TENDERS SHOULD BE DIRECTED TO, THE
DEPOSITARY. ALL NOTICES OF SOLICITED TENDERS MUST BE RECEIVED BY THE DEPOSITARY
WITHIN THREE BUSINESS DAYS AFTER THE EXPIRATION DATE.
 
          BENEFICIAL HOLDERS OF LESS THAN        SHARES (TENDERS ONLY)
 
<TABLE>
<CAPTION>
                                     ------------------------------------------------------------------
                                        TO BE COMPLETED BY SOLICITING           TO BE COMPLETED ONLY
- ----------------------------------------------------DEALER-------------------------BY-DEPOSITARY-------
                                                  NUMBER OF                  NUMBER OF
                                     SERIES OF     SHARES      VOI TICKET     SHARES        $ FEE
        BENEFICIAL OWNERS            PREFERRED    TENDERED      NUMBER*      ACCEPTED     PER SHARE
- -------------------------------------------------------------------------------------------------------
<S>                                  <C>          <C>          <C>           <C>          <C>       <C>
 Beneficial Owner No. 1
- -------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 2
- -------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 3
- -------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 4
- -------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 5
- -------------------------------------------------------------------------------------------------------
 Total
- -------------------------------------------------------------------------------------------------------
</TABLE>
<PAGE>   5
 
           BENEFICIAL HOLDERS OF        OR MORE SHARES (TENDERS ONLY)
 
<TABLE>
<CAPTION>
                                     -----------------------------------------------------------------
                                        TO BE COMPLETED BY SOLICITING           TO BE COMPLETED ONLY
                                                   DEALER                           BY-DEPOSITARY
- ------------------------------------------------------------------------------------------------------
                                                  NUMBER OF                  NUMBER OF
                                     SERIES OF     SHARES      VOI TICKET     SHARES        $ FEE
        BENEFICIAL OWNERS            PREFERRED    TENDERED      NUMBER*      ACCEPTED     PER SHARE
- ------------------------------------------------------------------------------------------------------
<S>                                  <C>          <C>          <C>           <C>          <C>       <C>
 Beneficial Owner No. 1
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 2                                                                               
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 3                                                                               
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 4                                                                               
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 5                                                                               
- ------------------------------------------------------------------------------------------------------
 Total                                                                                                
- ------------------------------------------------------------------------------------------------------
</TABLE>
        
* Complete if Shares delivered by book-entry transfer. Please submit a 
  separate VOI ticket for Shares tendered when the solicitation fee is to be
  directed to another Soliciting Dealer. At the time of tendering Shares in
  book-entry form, please indicate your request in the comments field. 
                
            PROXY FOR BENEFICIAL HOLDERS OF LESS THAN        SHARES
                                                                   
<TABLE>
<CAPTION>
                                                  ----------------------------------------------------
                                                    TO BE COMPLETED BY         TO BE COMPLETED ONLY   
                                                    SOLICITING-DEALER              BY DEPOSITARY      
- ------------------------------------------------------------------------------------------------------
                                                               NUMBER OF    NUMBER OF                 
                                                  SERIES OF     SHARES       SHARES        $ FEE      
              BENEFICIAL OWNERS                   PREFERRED      VOTED      ACCEPTED     PER SHARE    
- ------------------------------------------------------------------------------------------------------
<S>                                               <C>          <C>          <C>          <C>       <C>
 Beneficial Owner No. 1                                                                               
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 2                                                                               
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 3                                                                               
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 4                                                                               
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 5                                                                               
- ------------------------------------------------------------------------------------------------------
 Total                                                                                                
- ------------------------------------------------------------------------------------------------------
</TABLE>
 
              PROXY FOR BENEFICIAL HOLDERS OF       OR MORE SHARES
 
<TABLE>
<CAPTION>
                                                  ----------------------------------------------------
                                                    TO BE COMPLETED BY         TO BE COMPLETED ONLY
                                                    SOLICITING-DEALER              BY DEPOSITARY
- ------------------------------------------------------------------------------------------------------
                                                               NUMBER OF    NUMBER OF
                                                  SERIES OF     SHARES       SHARES        $ FEE
              BENEFICIAL OWNERS                   PREFERRED      VOTED      ACCEPTED     PER SHARE
- ------------------------------------------------------------------------------------------------------
<S>                                               <C>          <C>          <C>          <C>       <C>
 Beneficial Owner No. 1
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 2
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 3
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 4
- ------------------------------------------------------------------------------------------------------
 Beneficial Owner No. 5
- ------------------------------------------------------------------------------------------------------
 Total
- ------------------------------------------------------------------------------------------------------
</TABLE>
 
     All questions as to the validity, form and eligibility (including time of
receipt) of any Notice of Solicited Tenders and Proxies will be determined by
the Depositary, in its sole discretion, which determination will be final and
binding. Neither the Depositary nor any other person will be under any duty to
give notification of


<PAGE>   6
 
any defects or irregularities in any Notice of Solicited Tenders or incur any
liability for failure to give such notification.
 
     PLEASE COMPLETE THE SIGNATURE FORM ON THE LAST PAGE.
 
     DO NOT SEND STOCK CERTIFICATES WITH THIS FORM. YOUR STOCK CERTIFICATES MUST
BE SENT WITH THE LETTER OF TRANSMITTAL AND PROXY.
 
     The undersigned hereby confirms that: (i) it has complied with the
applicable requirements of the Securities Exchange Act of 1934, as amended, and
the applicable rules and regulations thereunder, in connection with such
solicitation; (ii) it is entitled to such compensation for such solicitation
under the terms and conditions of the Offer; (iii) in soliciting tenders of
Shares, it has used no soliciting materials other than those furnished by IPC or
ILN; and (iv) if it is a foreign broker or dealer not eligible for membership in
the NASD, it has agreed to conform to the NASD's Rules of Fair Practice in
making solicitations.


 
- ------------------------                    ----------------------------       
Firm Name                                   Address (including Zip Code)   
                                
- ------------------------                    ------------------------------
By:                                         Area Code and Telephone Number 
Title:





<PAGE>   7
 
                 DO NOT SEND STOCK CERTIFICATES WITH THIS FORM.
                 YOUR STOCK CERTIFICATES MUST BE SENT WITH THE
                        LETTER OF TRANSMITTAL AND PROXY
- --------------------------------------------------------------------------------
 
                     SOLICITATION FEE PAYMENT INSTRUCTIONS
 
 Issue check to:
 
 Name:
      -------------------------------------------------------------------------
                                 (Please Print)
 
 Address:
         ----------------------------------------------------------------------

 
 ------------------------------------------------------------------------------
                               (Include Zip Code)

 
 ------------------------------------------------------------------------------
                (Taxpayer Identification or Social Security No.)

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

<PAGE>   1
 
                              ILLINOVA CORPORATION
                           OFFER TO PURCHASE FOR CASH
           ANY AND ALL OUTSTANDING SHARES OF THE FOLLOWING SERIES OF
                           CUMULATIVE PREFERRED STOCK
                                       OF
                             ILLINOIS POWER COMPANY
 
283,290 SHARES, CUMULATIVE PREFERRED STOCK,
 4.08% SERIES AT A PURCHASE PRICE OF $     PER SHARE, CUSIP NUMBER 452092 20 8
 
167,720 SHARES, CUMULATIVE PREFERRED STOCK,
 4.20% SERIES AT A PURCHASE PRICE OF $     PER SHARE, CUSIP NUMBER 452092 30 7
 
136,000 SHARES, CUMULATIVE PREFERRED STOCK,
 4.26% SERIES AT A PURCHASE PRICE OF $     PER SHARE, CUSIP NUMBER 452092 40 6
 
134,400 SHARES, CUMULATIVE PREFERRED STOCK,
 4.42% SERIES AT A PURCHASE PRICE OF $     PER SHARE, CUSIP NUMBER 452092 50 5
 
176,000 SHARES, CUMULATIVE PREFERRED STOCK,
 4.70% SERIES AT A PURCHASE PRICE OF $     PER SHARE, CUSIP NUMBER 452092 60 4
 
241,700 SHARES, CUMULATIVE PREFERRED STOCK,
 7.75% SERIES AT A PURCHASE PRICE OF $     PER SHARE, CUSIP NUMBER 452092 79 4
 
       THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 5:00 P.M., NEW YORK
       CITY TIME, ON                     , UNLESS THE OFFER IS EXTENDED.
 
                                                                [STATEMENT DATE]
 
To Our Clients:
 
   
     Enclosed for your consideration are the Offer to Purchase and Proxy
Statement, dated [STATEMENT DATE] (the "Offer to Purchase and Proxy Statement"),
a Proxy and a separate Letter of Transmittal and Proxy for each series of
cumulative preferred stock listed above (each a "Series of Preferred" and the
holder thereof a "Preferred Shareholder") of Illinois Power Company, an Illinois
corporation ("IPC") and direct utility subsidiary of Illinova Corporation, an
Illinois corporation ("ILN"), of which you own shares. As to each Series of
Preferred, the Offer to Purchase and Proxy Statement, together with the
applicable Letter of Transmittal and Proxy, constitutes the "Offer" of ILN to
purchase any and all shares of the Series of Preferred ("Shares") at the
purchase price per Share listed above plus accrued and unpaid dividends for the
Shares tendered, net to the seller in cash, upon the terms and subject to the
conditions of the Offer. ILN will purchase any and all Shares validly tendered
and not withdrawn, upon the terms and subject to the conditions of the Offer.
The Offer for a Series of Preferred is not conditioned upon any minimum number
of Shares of such Series of Preferred being tendered and is independent of the
Offer for any other Series of Preferred. While Preferred Shareholders who wish
to tender their Shares pursuant to the Offer need not vote in favor of the
proposed amendment to IPC's Amended and Restated Articles of Incorporation, as
set forth in the Offer to Purchase and Proxy Statement (the "Proposed
Amendment"), the Offer is conditioned upon the Proposed Amendment being approved
and adopted at the Special Meeting (as defined in the Offer to Purchase and
Proxy Statement). IPC's Board of Directors recommends voting in favor of the
Proposed Amendment. In addition, Preferred Shareholders have the right to vote
for the Proposed Amendment regardless of whether they tender their Shares. If
the Proposed Amendment is adopted by IPC's shareholders, IPC will make a special
cash payment in the amount of $     per Share to each Preferred Shareholder of
record as of [RECORD DATE], provided that such Shares have not been tendered
pursuant to the Offer. See "Proposed Amendment and Proxy Solicitation", "Terms
of the Offer -- Certain Conditions of the Offer" and "Terms of the Offer --
Extension of Tender Period; Termination; Amendments" in the Offer to Purchase
and Proxy Statement.
    
<PAGE>   2
 
     WE ARE THE HOLDER OF RECORD OF SHARES HELD FOR YOUR ACCOUNT BUT NOT
REGISTERED IN YOUR NAME. A TENDER OR A VOTE OF SUCH SHARES CAN BE MADE ONLY BY
US AS THE HOLDER OF RECORD AND PURSUANT TO YOUR INSTRUCTIONS. ANY LETTER OF
TRANSMITTAL AND PROXY OR SEPARATE PROXY FURNISHED TO YOU IS SOLELY FOR YOUR
INFORMATION AND CANNOT BE USED BY YOU TO TENDER OR VOTE SHARES HELD BY US FOR
YOUR ACCOUNT.
 
     We request instructions as to whether you wish us to tender and/or vote any
or all of the Shares held by us for your account, upon the terms and subject to
the conditions set forth in the Offer.
 
     PLEASE READ THE FOLLOWING INFORMATION CAREFULLY:
 
     (1) The Offer is for any and all Shares outstanding as of [STATEMENT DATE].
The Offer for a Series of Preferred is independent of the Offer for any other
Series of Preferred.
 
     (2) The Offer and withdrawal rights will expire at 5:00 p.m., New York City
time, on [EXPIRATION DATE], unless the Offer is extended with respect to a
Series of Preferred. Your instructions to us should be forwarded to us in ample
time to permit us to submit a tender on your behalf by the expiration of the
Offer. If you would like to withdraw your Shares that we have tendered, you can
withdraw them so long as the Offer remains open or at any time after the
expiration of forty business days from the commencement of the Offer if such
tendered Shares have not been accepted for payment.
 
   
     (3) While Preferred Shareholders who wish to tender their Shares pursuant
to the Offer need not vote in favor of the Proposed Amendment, the Offer is
conditioned upon the Proposed Amendment being adopted at the Special Meeting.
    
 
   
     (4) Preferred Shareholders have the right to vote for the Proposed
Amendment regardless of whether they tender their Shares. If the Proposed
Amendment is adopted at the Special Meeting, IPC will make a special cash
payment in the amount of $  per Share to each Preferred Shareholder of record as
of [RECORD DATE], provided that such Shares have not been tendered pursuant to
the Offer.
    
 
     (5) Any stock transfer taxes applicable to the sale of Shares to ILN
pursuant to the Offer will be paid by ILN, except as otherwise provided in
Instruction 6 of the Letter of Transmittal and Proxy.
 
     NEITHER ILN, IPC, THEIR RESPECTIVE BOARDS OF DIRECTORS NOR ANY OF THEIR
RESPECTIVE OFFICERS MAKES ANY RECOMMENDATION TO ANY PREFERRED SHAREHOLDER AS TO
WHETHER TO TENDER ANY OR ALL SHARES. EACH PREFERRED SHAREHOLDER MUST MAKE HIS OR
HER OWN DECISION AS TO WHETHER TO TENDER SHARES AND, IF SO, HOW MANY SHARES TO
TENDER.
 
     If you wish to have us tender and/or vote any or all of your Shares held by
us for your account upon the terms and subject to the conditions set forth in
the Offer, please so instruct us by completing, executing, detaching and
returning to us the instruction form on the detachable part hereof. An envelope
to return your instructions to us is enclosed. If you authorize tender of your
Shares, all such Shares will be tendered unless otherwise specified on the
detachable part hereof. Your instructions should be forwarded to us in ample
time to permit us to submit a tender and/or vote on your behalf by the
expiration of the Offer or the Special Meeting, as applicable.
 
     The Offer is being made to all holders of Shares. ILN is not aware of any
state where the making of the Offer is prohibited by administrative or judicial
action pursuant to a valid state statute. If ILN becomes aware of any valid
state statute prohibiting the making of the Offer, ILN will make a good faith
effort to comply with such statute. If, after such good faith effort, ILN cannot
comply with such statute, the Offer will not be made to, nor will tenders be
accepted from or on behalf of, holders of Shares in such state. In those
jurisdictions where the securities, blue sky or other laws require the Offer to
be made by a licensed broker or dealer, the Offer shall be deemed to be made on
behalf of ILN by the Dealer Manager (as defined in the Offer) or one or more
registered brokers or dealers licensed under the laws of such jurisdictions.
<PAGE>   3
 
                                  INSTRUCTIONS
                   WITH RESPECT TO OFFER TO PURCHASE FOR CASH
                         ANY AND ALL OUTSTANDING SHARES
                                       OF
                              THE PREFERRED STOCK
   
                                       OF
    
   
    
   
                             ILLINOIS POWER COMPANY
    
   
                                       BY
    
   
                              ILLINOVA CORPORATION
    
                             AND PROXY SOLICITATION
                                       BY
 
                             ILLINOIS POWER COMPANY
 
   
     The undersigned acknowledge(s) receipt of your letter and the enclosed
Offer to Purchase and Proxy Statement, dated [STATEMENT DATE], Proxy and a
separate Letter of Transmittal and Proxy for each series of preferred stock of
Illinois Power Company (each a "Series of Preferred") in which the undersigned
owns shares (as to each Series of Preferred, the Offer to Purchase and Proxy
Statement, together with the applicable Letter of Transmittal and Proxy,
constitutes the "Offer") in connection with the invitation of Illinova
Corporation ("ILN") to the holders of each Series of Preferred to tender any and
all of their shares of a Series of Preferred ("Shares") for purchase at the
purchase price per Share listed on the front cover of the Offer to Purchase and
Proxy Statement plus accrued and unpaid dividends for the Shares tendered, net
to the seller in cash, upon the terms and subject to the conditions of the
Offer, and in connection with the proxy solicitation being conducted by the
Board of Directors of Illinois Power Company.
    
 
     This will instruct you to tender to ILN the number of Shares indicated
below (or, if no number is indicated below, all Shares) which are held by you
for the account of the undersigned, upon the terms and subject to the conditions
of the Offer.
 
                              SERIES OF PREFERRED
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
- ------------------------------------------------------
                        NUMBER OF SHARES TO BE TENDERED*
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
- ------------------------------------------------------
 
     You are further instructed to vote as designated hereunder in respect of
the Proposed Amendment all shares which the undersigned is entitled to vote at
the Special Meeting:**
 
         [ ]  FOR               [ ]  AGAINST               [ ]  ABSTAIN
 
                                                         SIGN HERE
 
                                            ------------------------------------
                                                       Signature(s):
 
                                            ------------------------------------
                                                          Name(s):
 
                                            ------------------------------------
                                                          Address:
 
                                            ------------------------------------
                                                Social Security or Taxpayer
                                                    Identification No.:
 
                                            Dated:  , 1998
- -------------------------
 * By executing and returning these Instructions, unless otherwise indicated, it
   will be assumed that all Shares held by us for your account are to be
   tendered.
** By executing and returning these Instructions, unless otherwise indicated, it
   will be assumed that all Shares held by us for your account are to be voted
   FOR the Proposed Amendment.

<PAGE>   1
 
                                                                [STATEMENT DATE]
 
Dear Preferred Shareholder:
 
   
     Enclosed for your consideration is information regarding two proposals
relating to Illinois Power Company ("IPC") and your investment in shares of
preferred stock of IPC. First, the Board of Directors of IPC is soliciting
proxies for use at a special meeting of shareholders to be held on [EXPIRATION
DATE] at 4:30 p.m., New York City time, at the principal office of IPC located
at 500 South 27th Street, Decatur, Illinois 62525, to amend IPC's Articles of
Incorporation to remove a provision which limits the ability of IPC to issue or
assume unsecured indebtedness (the "Proposed Amendment"). Second, IPC's parent
holding company, Illinova Corporation ("ILN") is offering to purchase all of the
outstanding shares of preferred stock of IPC (the "Offer to Purchase"). If the
Proposed Amendment is adopted at the Special Meeting, you will be entitled to
receive from IPC a special cash payment ("Special Cash Payment") of $       for
each share you held as of [RECORD DATE], provided your shares have not been
tendered in the Offer to Purchase. Both the Offer to Purchase and the proxy
solicitation for the Proposed Amendment are more fully described in the enclosed
Offer to Purchase and Proxy Statement.
    
 
     In connection with the Offer to Purchase and the proxy solicitation for the
Proposed Amendment, we ask that you consider the following:
 
     1. If you wish to accept the Offer to Purchase and tender your shares:
 
      - Complete, sign, and date the appropriate Letter of Transmittal and Proxy
        in accordance with the instructions in the Letter of Transmittal and
        Proxy.
 
   
      - Enclose your completed Letter of Transmittal and Proxy and related
        certificates for your shares in the envelope addressed to First Chicago
        Trust Company of New York at the appropriate address set forth on the
        back cover of the enclosed Offer to Purchase and Proxy Statement.
    
 
      - Please note that the Letter of Transmittal and Proxy should be used to
        vote on the Proposed Amendment if you are tendering Shares.
 
      PLEASE BE AWARE THAT THE OFFER TO PURCHASE IS CONDITIONED UPON ADOPTION OF
      THE PROPOSED AMENDMENT AT THE SPECIAL MEETING. A FAILURE TO RETURN A PROXY
      IS A VOTE AGAINST THE PROPOSED AMENDMENT. (Please also note that you
      cannot receive a Special Cash Payment if you tender your shares in the
      Offer to Purchase).
 
     2. If you wish to receive the Special Cash Payment:
 
   
      - Please vote, sign and date the [PROXY CARD COLOR] Proxy card. PLEASE BE
        AWARE THAT THE PROPOSED AMENDMENT MUST BE ADOPTED IN ORDER FOR YOU TO
        RECEIVE A SPECIAL CASH PAYMENT. A FAILURE TO RETURN A PROXY IS A VOTE
        AGAINST THE PROPOSED AMENDMENT. (Please also note that you cannot
        receive a Special Cash Payment if you tender your shares in the Offer to
        Purchase.)
    
 
   
      - Enclose your completed Proxy card in the envelope addressed to First
        Chicago Trust Company of New York at the appropriate address set forth
        on the back cover of the enclosed Offer to Purchase and Proxy Statement.
    
 
     If you have any questions on how to tender or vote your shares, please
contact:
 
                          MacKenzie Partners, Inc. at:
                           (800) 322-2885 (Toll Free)
                         (212) 929-5500 (Call Collect)


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