================================================================================


                         FIRST SUPPLEMENTAL INDENTURE


                                    BETWEEN


                                AON CORPORATION


                                      AND


                             THE BANK OF NEW YORK


                         DATED AS OF JANUARY 13, 1997



          8.205% JUNIOR SUBORDINATED DEFERRABLE INTEREST DEBENTURES,
                              DUE JANUARY 1, 2027


================================================================================

 
                               TABLE OF CONTENTS

 
 
                                                                                     Page
                                                                                   
                                   ARTICLE I
                                  DEFINITIONS

Section 1.01.  Definition of Terms.................................................      2

                                   ARTICLE 2
                      GENERAL TERMS AND CONDITIONS OF THE
                        JUNIOR SUBORDINATED DEBENTURES

Section 2.01.  Designation and Principal Amount....................................      5
Section 2.02.  Stated Maturity.....................................................      5
Section 2.03.  Form and Payment; Minimum Transfer Restriction......................      5
Section 2.04.  Exchange and Registration of Transfer of Junior Subordinated
                Debentures; Restrictions on Transfers; Depositary..................      6
Section 2.05.  Interest............................................................      9

                                   ARTICLE 3
                       REDEMPTION AND PREPAYMENT OF THE
                        JUNIOR SUBORDINATED DEBENTURES

Section 3.01.  Tax Event or Investment Company Event Prepayment....................     10
Section 3.02.  Notice of Prepayment................................................     10

                                   ARTICLE 4
                     EXTENSION OF INTEREST PAYMENT PERIOD

Section 4.01.  Extension of Interest Payment Period................................     10
Section 4.02.  Notice of Extension.................................................     11

                                   ARTICLE 5
                                   EXPENSES

Section 5.01.  Payment of Expenses.................................................     11
Section 5.02.  Payment upon Resignation or Removal.................................     12

                                   ARTICLE 6
                     FORM OF JUNIOR SUBORDINATED DEBENTURE

Section 6.01.  Form of Junior Subordinated Debenture...............................     12


                                      -i-

 
 
                                                                                      
                                   ARTICLE 7
               ORIGINAL ISSUE OF JUNIOR SUBORDINATED DEBENTURES

Section 7.01.  Original Issue of Junior Subordinated Debentures....................     13

                                   ARTICLE 8
                           EXCHANGE OF SECURITIES

Section 8.01.  Mandatory Tender in Exchange Offer..................................     13

                                   ARTICLE 9
                                 MISCELLANEOUS

Section 9.01.  Ratification of Indenture; First Supplemental Indenture Controls....     13
Section 9.02.  Trustee Not Responsible for Recitals................................     14
Section 9.03.  Governing Law.......................................................     14
Section 9.04.  Separability........................................................     14
Section 9.05.  Counterparts........................................................     14


                                     -ii-




 
          FIRST SUPPLEMENTAL INDENTURE, dated as of January 13, 1997 (the "First
                                                                           -----
Supplemental Indenture"), between Aon Corporation, a Delaware corporation (the
- ----------------------                                                        
"Company"), and The Bank of New York, as trustee (the "Trustee") under the
- --------                                               -------            
Indenture dated as of January 13, 1997 between the Company and the Trustee (the
"Base Indenture" and, together with this First Supplemental Indenture, the
 --------------                                                           
"Indenture").
- ----------   

          WHEREAS, the Company executed and delivered the Base Indenture to the
Trustee to provide for the future issuance of the Company's unsecured junior
subordinated debentures (the "Debentures") to be issued from time to time in one
                              ----------                                        
or more series as might be determined by the Company under the Indenture, in an
unlimited aggregate principal amount which may be authenticated and delivered as
provided in the Base Indenture;

          WHEREAS, pursuant to the terms of the Base Indenture, the Company
desires to provide for the establishment of two separate series of its
Debentures, both to be known as its 8.205% Junior Subordinated Deferrable
Interest Debentures due January 1, 2027 (collectively, the "Junior Subordinated
                                                            -------------------
Debentures"), the form and substance of such Junior Subordinated Debentures and
- ----------                                                                     
the terms, provisions and conditions thereof to be set forth as provided in the
Base Indenture and this First Supplemental Indenture;

          WHEREAS, the Company desires that (x) the first series of Junior
Subordinated Debentures (the "Private Debentures") be originally issued on
                              ------------------                          
January 13, 1997 pursuant to the Indenture, the Purchase Agreement (as defined
below) and the Trust Agreement (as defined in Section 1.01), and (y) the second
series of Junior Subordinated Debentures (the "Exchange Debentures") be issuable
                                               -------------------              
upon surrender of and in exchange for the Private Debentures pursuant to Section
8.01;

          WHEREAS, Aon Capital A, a Delaware statutory business trust (the
"Trust"), has offered to the purchasers (the "Purchasers") named in Schedule I
                                              ----------                      
to the Purchase Agreement (the "Purchase Agreement") dated January 8, 1997 among
                                ------------------                              
the Purchasers, the Trust and the Company in a private placement $800,000,000
aggregate liquidation amount of its 8.205% Capital Securities (the "Capital
                                                                    -------
Securities"), representing undivided beneficial interests in the assets of the
- ----------                                                                    
Trust and proposes to invest the proceeds from the sale of the Capital
Securities, together with the proceeds of the sale by the Trust to the Company
of $24,000,000 aggregate liquidation amount of its Common Securities, in
$824,000,000 aggregate principal amount of the Junior Subordinated Debentures;
and

          WHEREAS, the Company has requested that the Trustee execute and
deliver this First Supplemental Indenture and all requirements necessary to make
this First Supplemental Indenture a valid instrument in accordance with its
terms, and to make the Junior Subordinated Debentures, when executed by the
Company and authenticated and delivered by the Trustee, the valid obligations of
the Company, have been performed, and the execution and delivery of this First
Supplemental Indenture has been duly authorized in all respects.

          NOW, THEREFORE, in consideration of the purchase and acceptance of the
Junior Subordinated Debentures by the Purchasers, and for the purpose of setting
forth, as provided in the Base Indenture, the form and substance of the Junior
Subordinated Debentures and the terms, provisions and conditions thereof, the
Company covenants and agrees with the Trustee as follows:

 
                                   ARTICLE I

                                  DEFINITIONS

          SECTION 1.01.  Definition of Terms.  For all purposes of this First
Supplemental Indenture, except as otherwise expressly provided or unless the
context otherwise requires:

          (a)  the terms which are defined in the Base Indenture have the same
     meanings when used in this First Supplemental Indenture;

          (b)  the terms defined in this Article have the meanings assigned to
     them in this Article and include the plural as well as the singular;

          (c)  all other terms used herein which are defined in the Trust
     Indenture Act of 1939, whether directly or by reference therein, have the
     meanings assigned to them therein;

          (d)  all accounting terms not otherwise defined herein have the
     meanings assigned to them in accordance with generally accepted accounting
     principles in the United States of America, and, except as otherwise herein
     expressly provided, the term "generally accepted accounting principles"
     with respect to any computation required or permitted hereunder shall mean
     such accounting principles as are generally accepted in the United States
     of America at the date of such computation; provided, that when two or more
     principles are so generally accepted, it shall mean that set of principles
     consistent with those in use by the Company;

          (e)  a reference to a Section or Article is to a Section or Article of
     this First Supplemental Indenture unless otherwise stated;

          (f)  the words "herein," "hereof" and "hereunder" and other words of
     similar import refer to this First Supplemental Indenture as a whole and
     not to any particular Article, Section or other subdivision;

          (g)  headings are for convenience of reference only and do not affect
     interpretation;

          (h)  the term "prepayment" as used herein means "redemption" as such
     term is used in the Base Indenture; and

          (i)  the following terms have the meanings given to them in the Trust
     Agreement: (i) Additional Distributions, (ii) Administrative Trustee, (iii)
     Affiliate, (iv) Capital Securities Certificate, (v) Capital Securities
     Exchange and Registration Rights Agreement, (vi) Cedel, (vii) Debenture
     Exchange and Registration Rights Agreement, (viii) Delaware Trustee, (ix)
     Distributions, (x) Euroclear, (xi) Exchange Act, (xii) Institutional
     Accredited Investor, (xiii) Minimum Transfer Legend, (xiv) 144A Global
     Security, (xv) PORTAL Market, (xvi) Private Placement Legend, (xvii)
     Property 

                                      -2-

 
     Trustee, (xviii) QIB, (xix) Registration Rights Agreements, (xx) Regulation
     S, (xxi) Regulation S Global Security, (xxii) Restricted Period, (xxiii)
     Rule 144, (xxiv) Rule 144A, (xxv) Rule 144(k), (xxvi) Securities Act,
     (xxvii) Transfer Restriction Termination Date, and (xxviii) Trust Security.

          "Additional Interest" has the meaning specified in Section 2.05.
           -------------------                                            

          "Adjusted Treasury Rate" means, with respect to any prepayment date,
           ----------------------                                             
the Treasury Rate plus (i) 1.00% if such prepayment date occurs on or before
January 1, 1998 or (ii) 0.50% if such prepayment date occurs after January 1,
1998.

          "Capital Securities" has the meaning specified in the fourth recital
           ------------------                                                 
to this First Supplemental Indenture.

          "Comparable Treasury Issue" means with respect to any prepayment date
           -------------------------                                           
the United States Treasury security selected by the Quotation Agent as having a
maturity comparable to the Remaining Life that would be utilized, at the time of
selection and in accordance with customary financial practice, in pricing new
issues of corporate debt securities of comparable maturity to the Remaining
Life.  If no United States Treasury security has a maturity which is within a
period from three months before to three months after January 1, 2027, the two
most closely corresponding United States Treasury securities shall be used as
the Comparable Treasury Issue, and the Treasury Rate shall be interpolated or
extrapolated on a straight-line basis, rounding to the nearest month using such
securities.

          "Comparable Treasury Price" means, with respect to any prepayment
           -------------------------                                       
date, (A) the average of five Reference Treasury Dealer Quotations for such
prepayment date, after excluding the highest and lowest such Reference Treasury
Dealer Quotations, or (B) if the Trustee obtains fewer than three such Reference
Treasury Dealer Quotations, the average of all such Quotations.

          "Coupon Rate" has the meaning specified in Section 2.05(a).
           -----------                                               

          "Debentures" has the meaning specified in the first recital to this
           ----------                                                        
First Supplemental Indenture.

          "Definitive Debenture Certificates" means Debentures issued in
           ---------------------------------                            
definitive, fully registered form.

          "Event Prepayment Price" has the meaning specified in Section 3.01.
           ----------------------                                            

          "Exchange Debentures" has the meaning specified in the third recital
           -------------------                                                
to this First Supplemental Indenture.

          "Extension Period" has the meaning specified in Section 4.01.
           ----------------                                            

          "Global Debenture" has the meaning specified in Section 2.04(a).
           ----------------                                               

          "Global Private Debenture" has the meaning specified in Section
           ------------------------                                      
2.04(d).

                                      -3-

 
          "Interest Payment Date" has the meaning specified in Section  2.05.
           ---------------------                                             

          "Junior Subordinated Debentures" has the meaning specified in the
           ------------------------------                                  
second recital to this First Supplemental Indenture and, unless the context
otherwise requires, shall include any Exchange Debentures to be issued and
exchanged for any Private Debentures.

          "Liquidation Amount" means the stated amount of $1,000 per Capital
           ------------------                                               
Security.

          "144A Global Debenture" has the meaning specified in Section 2.04(c).
           ---------------------                                               

          "Private Debentures" has the meaning specified in the third recital to
           ------------------                                                   
this First Supplemental Indenture.

          "Purchase Agreement" has the meaning specified in the fourth recital
           ------------------                                                 
to this First Supplemental Indenture.

          "Purchasers" has the meaning specified in the fourth recital to this
           ----------                                                         
First Supplemental Indenture.

          "Quotation Agent" means Morgan Stanley & Co. Incorporated.
           ---------------                                          

          "Record Date" has the meaning specified in Section 2.05(a).
           -----------                                               

          "Reference Treasury Dealer" means (i) Morgan Stanley & Co.
           -------------------------                                
Incorporated and Goldman, Sachs & Co. and their respective successors; provided,
however, that if any of the foregoing shall cease to be a primary U.S.
Government securities dealer in New York City (a "Primary Treasury Dealer"), the
                                                  -----------------------       
Company shall substitute therefor another Primary Treasury Dealer; and (ii) any
other Primary Treasury Dealer selected by the Trustee after consultation with
the Company.

          "Reference Treasury Dealer Quotations" means, with respect to each
           ------------------------------------                             
Reference Treasury Dealer and any prepayment date, the average, as determined by
the Trustee, of the bid and asked prices for the Comparable Treasury Issue
(expressed in each case as a percentage of its principal amount) quoted in
writing to the Trustee by such Reference Treasury Dealer at 5:00 p.m. on the
third Business Day preceding such prepayment date.

          "Regulation S Global Debenture" has the meaning specified in Section
           -----------------------------                                      
2.04(d).

          "Remaining Life" has the meaning specified in Section 3.01.
           --------------                                            

          "Restricted Period" means the period of 40 consecutive days beginning
           -----------------                                                   
on and including the later of (x) the day on which the offering of the Capital
Securities commences or (y) the Closing Date.

          "Special Interest" has the meaning specified in Section 2.05(c).
           ----------------                                               

                                      -4-

 
          "Treasury Rate" means (i) the yield, under the heading which
           -------------                                              
represents the average for the immediately prior week, appearing in the most
recently published statistical release designated "H.R.(519)" or any successor
publication which is published weekly by the Federal Reserve and which
establishes yields on actively traded United States Treasury securities adjusted
to constant maturity under the caption "Treasury Constant Maturities," for the
maturity corresponding to the Remaining Life (if no maturity is within three
months before or after the Remaining Life, yields for the two published
maturities most closely corresponding to the Remaining Life shall be determined
and the Treasury Rate shall be interpolated or extrapolated from such yields on
a straight-line basis, rounding to the nearest month) or (ii) if such release
(or any successor release) is not published during the week preceding the
calculation date or does not contain such yields, the rate per annum equal to
the semi-annual equivalent yield to maturity of the Comparable Treasury Issue,
calculated using a price for the Comparable Treasury Issue (expressed as a
percentage of its principal amount) equal to the Comparable Treasury Price for
such prepayment date.  The Treasury Rate shall be calculated on the third
Business Day preceding the prepayment date.

          "Trust" has the meaning specified in the fourth recital to this First
           -----                                                               
Supplemental Indenture.

          "Trust Agreement" means the Amended and Restated Trust Agreement dated
           ---------------                                                      
as of January 13, 1997 among the Company, as Depositor, The Bank of New York, as
Property Trustee, The Bank of New York (Delaware), as Delaware Trustee, the
Administrative Trustees named therein and the holders, from time to time, of
undivided beneficial interests in the assets of the Trust.


                                   ARTICLE 2

       GENERAL TERMS AND CONDITIONS OF THE JUNIOR SUBORDINATED DEBENTURES

          SECTION 2.01.  Designation and Principal Amount.  There is hereby
authorized two series of Debentures, both to be designated the "8.205% Junior
Subordinated Deferrable Interest Debentures due January 1, 2027," and each
limited in aggregate principal amount to $824,000,000, which amount shall be as
set forth in any written orders of the Company for the authentication and
delivery of Junior Subordinated Debentures pursuant to Section 2.01 of the Base
Indenture and Section 7.01(a).

          SECTION 2.02.  Stated Maturity.  The Stated Maturity of the Junior
Subordinated Debentures is January 1, 2027, and shall not be subject to
extension.

          SECTION 2.03.  Form and Payment; Minimum Transfer Restriction.  (a)
The Debentures shall be issued to the Trust in fully registered definitive form
without coupons in minimum denominations of $1,000 and integral multiples of
$1,000 in excess thereof.  Principal and interest on the Junior Subordinated
Debentures issued in definitive form will be payable, the transfer of such
Junior Subordinated Debentures will be registrable and such Junior Subordinated
Debentures will be exchangeable for Junior Subordinated Debentures bearing
identical terms and provisions at the principal corporate trust office of the
Trustee; provided, however, that payment of interest may be made at the option
         --------  -------    
of the Company by check mailed to the registered holder at such address as shall
appear in the Register. Notwithstanding the foregoing, so long as the registered
holder of any Junior Subordinated






                                      -5-

 
Debentures is the property Trustee, the payment of the principal of and interest
(including Additional interest, Special Interest and Additional Tax Sums, if
any) on such Junior Subordinated Debentures held by the Property Trustee will be
made at such place and to such account as may be designated by the Property
Trustee. The Register for the Junior Subordinated shall be kept at the principal
corporate trust office of the Trustee is hereby appointed registrar for the
Junior Subordinated Debentures.

          (b) The Junior Subordinated Debentures may be transferred or exchanged
only in minimum denominations of $100,000 and integral multiples of $1,000 in
excess thereof, and any attempted transfer, sale or other disposition of Junior
Subordinated Debentures in a denomination of less than $100,000 shall be deemed
to be void and of no legal effect whatsoever.

          SECTION 2.04.  Exchange and Registration of Transfer of Junior
Subordinated Debentures; Restrictions on Transfers; Depositary.  If distributed
to holders of Capital Securities pursuant to Section 9.4 of the Trust Agreement,
the Junior Subordinated Debentures will be issued to such holders in the same
form as the Capital Securities that such Junior Subordinated Debentures replace
in accordance with the following procedures:

          (a) So long as Junior Subordinated Debentures are eligible for book-
entry settlement with the Depositary, or unless required by law, all Junior
Subordinated Debentures that are so eligible will be represented by one or more
Junior Subordinated Debentures in global form (a "Global Debenture") registered
                                                  ----------------             
in the name of the Depositary or the nominee of the Depositary.  Except as
provided in Section 2.04(i) below, beneficial owners of a Global Debenture shall
not be entitled to have Definitive Debenture Certificates registered in their
names, will not receive or be entitled to receive physical delivery of
Definitive Debenture Certificates and will not be registered holders of such
Global Debentures; provided, however, that a Definitive Debenture Certificate
shall be issued upon any transfer of a beneficial interest in a Global Private
Debenture to the Company or an Affiliate of the Company and no Definitive
Debenture Certificate, or portion thereof, in respect of which the Company or an
Affiliate of the Company held any beneficial interest shall be resold,
retransferred or included in any Private Global Security until such Private
Debenture is freely tradeable in accordance with Rule 144(k) or exchanged for an
Exchange Debenture.

          (b) The transfer and exchange of beneficial interests in Global
Debentures shall be effected through the Depositary in accordance with the
Indenture and the procedures and standing instructions of the Depositary and the
Trustee shall make appropriate endorsements to reflect increases or decreases in
principal amounts of such Global Debentures.

          (c) Private Debentures that are distributed in replacement of Private
Capital Securities represented by a 144A Global Security will be represented by
a global Private Debenture (a "144A Global Debenture").  Transfers of beneficial
                               ---------------------                            
interests in a 144A Global Debenture will be subject to the restrictions on
transfer contained in the Private Placement Legend and the Minimum Transfer
Legend.

          (d) Private Debentures that are distributed in replacement of Private
Capital Securities represented by a Regulation S Global Security will be
represented by a global Private Debenture (a "Regulation S Global Debenture",
                                              -----------------------------  
and together with the 144A Global Debenture, the 

                                      -6-

 
"Global Private Debenture"). Prior to the expiration of the Restricted Period,
- ------------------------    
interests in a Regulation S Global Debenture may only be held by the
Depositary's participants in the name of a nominee of Euroclear and Cedel. After
the expiration of the Restricted Period, transfers of beneficial interests in a
Regulation S Global Debenture will not be subject to any restrictions other than
the restrictions contained in the Minimum Transfer Legend. After the expiration
of the Restricted Period, beneficial interests in the Regulation S Global
Debenture may be held by the Depositary's participants other than in the name of
a nominee of Euroclear and Cedel.

          (e)  Private Debentures that are distributed in replacement of
Definitive Capital Securities Certificates will be represented by Definitive
Debenture Certificates and transfers will be subject to the Private Placement
Legend, the Minimum Transfer Legend and the requirements of Section 2.04(g)(1)
or (g)(2).

          (f)  Exchange Debentures that are distributed in replacement of
Exchange Capital Securities will be represented by a Global Debenture or in such
other form as the Trustee may direct and will bear the Minimum Transfer Legend.

          (g)  Unless and until the earlier of (i) the date upon which Private
Debentures are exchanged for Exchange Debentures or (ii) the Transfer
Restriction Termination Date:

               (1)  Definitive to Definitive Transfers.  Any transfer of a
     Definitive Debenture Certificate shall be registered upon the Register only
     upon receipt by the Trustee of such Definitive Debenture Certificate
     accompanied by a duly completed and executed assignment in the form of
     Exhibit A-1 and, in the case of a transfer to an Institutional Accredited
     Investor, upon the receipt by the Trustee of a written certificate in the
     form of Exhibit B (or other certificates, legal opinions or other
     information as the Company may reasonably request to confirm that such
     transfer is exempt from the registration requirements of the Securities
     Act);

               (2)  Definitive into a 144A Global Debenture.  So long as Private
     Debentures are eligible for book-entry settlement with the Depositary or
     unless otherwise required by law, upon any transfer of a Definitive
     Debenture Certificate to a QIB in accordance with Rule 144A or to a non-
     U.S. Person in accordance with Regulation S, and upon receipt of the
     Definitive Debenture Certificate being so transferred, accompanied by a
     duly completed and executed assignment in the form attached hereto as
     Exhibit A-1, the Trustee, on behalf of the Trust, shall make an endorsement
     on any 144A Global Debenture or any Regulation S Global Debenture, as the
     case may be, to reflect an increase in such Global Debenture and the
     Trustee, on behalf of the Trust, shall cancel such Definitive Debenture
     Certificate;

               (3)  144A Global Debenture into Regulation S Global Debenture.
     Any transfer in accordance with Rule 904 of Regulation S of a beneficial
     interest in a 144A Global Debenture to a transferee that takes delivery in
     the form of a beneficial interest in a Regulation S Global Debenture shall
     be reflected by an increase in a Regulation S Global Debenture and a
     corresponding decrease in the 144A Global Debenture only upon receipt by
     the Trustee of a written certificate in the form of Exhibit C (or such
     other certifications, legal opinions or other information as the Company
     may reasonably request to confirm that such transfer is being made pursuant
     to Rule 904); and

                                      -7-

 
               (4)  Regulation S Global Debenture into 144A Global Debenture.
     Any transfer of a beneficial interest in a Regulation S Global Debenture to
     a transferee that takes delivery in the form of a beneficial interest in a
     144A Global Debenture shall be reflected by an increase in the 144A Global
     Debenture and a corresponding decrease in the Regulation S Global Debenture
     and, prior to the expiration of the Restricted Period, only upon receipt by
     the Trustee of a written certificate in the form of Exhibit D (or such
     other certifications, legal opinions or other information as the Company
     may reasonably require).

          (h)  Any Global Debenture may be endorsed with or have incorporated in
the text thereof such legends or recitals or changes not inconsistent with the
provisions of the Indenture as may be required by the Depositary, by any
national securities exchange or by the National Association of Securities
Dealers, Inc. in order for the Private Debentures to be tradeable on the PORTAL
Market or as may be required for the Private Debentures to be tradeable on any
other market developed for trading of securities pursuant to Rule 144A or
required to comply with any applicable law or any regulation thereunder or with
the rules and regulations of any securities exchange upon which the Junior
Subordinated Debentures may be listed or traded or to conform with any usage
with respect thereto, or to indicate any special limitations or restrictions to
which any particular Junior Subordinated Debentures are subject.

          (i)  Notwithstanding any other provisions of the Indenture (other than
the provisions set forth in this Section 2.04(i)), a Global Debenture may not be
exchanged in whole or in part for Junior Subordinated Debentures registered, and
no transfer of a Global Debenture may be registered, in the name of any person
other than the Depositary or a nominee thereof unless (i) such Depositary (A)
has notified the Company that it is unwilling or unable to continue as
Depositary for such Global Debenture or (B) has ceased to be a clearing agency
registered as such under the Exchange Act and no successor Depositary has been
appointed by the Company within 90 days after its receipt of such notice or its
becoming aware of such ineligibility, (ii) there shall have occurred and be
continuing an Event of Default, or any event which after notice or lapse of time
or both would be an Event of Default under the Indenture, with respect to such
Debenture, or (iii) the Company, in its sole discretion, instructs the Trustee
to exchange such Global Debenture for a Junior Subordinated Debenture that is
not a Global Debenture (in which case such exchange shall be effected by the
Trustee).

          The Depositary shall be a clearing agency registered under the
Exchange Act.  The Company initially appoints The Depository Trust Company to
act as Depositary with respect to the Global Debentures.  Initially, any Global
Debentures shall be registered in the name of Cede & Co., as the nominee of the
Depositary, and deposited with the Trustee as custodian for Cede & Co.

          Definitive Junior Subordinated Debentures issued in exchange for all
or a part of a Global Debenture pursuant to this Section 2.04(i) shall be
registered in such names and in such authorized denominations as the Depositary,
pursuant to instructions from its direct or indirect participants or otherwise,
shall instruct the Trustee.  Upon execution and authentication, the Trustee
shall deliver such definitive Junior Subordinated Debentures to the person in
whose names such definitive Junior Subordinated Debentures are so registered.

          So long as Junior Subordinated Debentures are represented by one or
more Global Debentures, (i) the registrar for the Junior Subordinated Debentures
and the Trustee shall be entitled to 

                                      -8-

 
deal with the clearing agency for all purposes of the Indenture relating to such
Global Debentures as the sole holder of the Junior Subordinated Debentures
evidenced by such Global Debentures and shall have no obligations to the holders
of beneficial interests in such Global Debentures; and (ii) the rights of the
holders of beneficial interests in such Global Debentures shall be exercised
only through the clearing agency and shall be limited to those established by
law and agreements between such holders and the clearing agency and/or the
participants in the clearing agency.

          At such time as all interests in a Global Debenture have been
redeemed, exchanged, repurchased or canceled, such Global Debenture shall be,
upon receipt thereof, canceled by the Trustee in accordance with standing
procedures and instructions of the Depositary.  At any time prior to such
cancellation, if any interest in a Global Debenture is exchanged for definitive
Junior Subordinated Debentures, redeemed by the Company pursuant to Article 3 or
canceled, or transferred for part of a Global Debenture, the principal amount of
such Global Debenture shall, in accordance with the standing procedures and
instructions of the Depositary be reduced or increased, as the case may be, and
an endorsement shall be made on such Global Debenture by, or at the direction
of, the Trustee to reflect such reduction or increase.

          SECTION 2.05.  Interest.  (a)  Each Junior Subordinated Debenture will
bear interest at the rate of 8.205% per annum (the "Coupon Rate") from January
                                                    -----------               
1, 1997 until the principal thereof becomes due and payable, and will bear
interest on any overdue principal at the Coupon Rate and (to the extent that
payment of such interest is enforceable under applicable law) on any overdue
installment of interest at the Coupon Rate ("Additional Interest"), compounded
                                             -------------------              
semiannually, payable (subject to the provisions of Article 4) semiannually in
arrears on the 1st day of January and July of each year (each, an "Interest
                                                                   --------
Payment Date"), commencing on July 1, 1997, to the Person in whose name such
- ------------                                                                
Junior Subordinated Debenture is registered, subject to certain exceptions, at
the close of business on the Record Date next preceding such Interest Payment
Date.  The "Record Date" for payment of interest will be the Business Day next
            -----------                                                       
preceding the Interest Payment Date, unless such Junior Subordinated Debenture
is registered to a holder other than the Property Trustee or a nominee of the
Depositary, in which case the Record Date for payment of interest will be the
fifteenth day of the calendar month next preceding the applicable Interest
Payment Date or, if such fifteenth day of the month is not a Business Day, then
the Business Day next preceding such day.  Until liquidation, if any, of the
Trust, each Junior Subordinated Debenture will be held in the name of the
Property Trustee in trust for the benefit of the holders of the Trust
Securities.

          (b)  The amount of interest payable for any period will be computed on
the basis of a 360-day year of twelve 30-day months.  In the event that any date
on which interest is payable on the Junior Subordinated Debentures is not a
Business Day, then payment of interest payable on such date will be made on the
next succeeding day which is a Business Day (and without any interest or other
payment in respect of any such delay), in each case with the same force and
effect as if made on the date such payment was originally payable.

          (c)  If the Company does not comply with certain of its obligations
under the Registration Rights Agreements, the Private Debentures shall, in
accordance with Section 2(c) of the Capital Securities Exchange and Registration
Rights Agreement and Section 2(c) of the Debenture Exchange and Registration
Rights Agreement, bear additional interest ("Special Interest") in addition to
                                             ----------------                 
the interest provided for in Section 2.05(a).

                                      -9-

 
                                   ARTICLE 3

        REDEMPTION AND PREPAYMENT OF THE JUNIOR SUBORDINATED DEBENTURES

          SECTION 3.01.   Tax Event or Investment Company Event Prepayment.  If
a Tax Event or Investment Company Event shall occur and be continuing, the
Company may, at its option, prepay the Junior Subordinated Debentures in whole
(but not in part) at any time within 90 days of the occurrence of such Tax Event
or Investment Company Event at a prepayment price (the "Event Prepayment Price")
                                                        ----------------------  
equal to the greater of (i) 100% of the principal amount of such Junior
Subordinated Debentures or (ii) as determined by the Quotation Agent, an amount
equal to the sum of the present value of 100% of the principal amount that would
be payable on January 1, 2027, together with the present values of scheduled
payments of interest from the prepayment date to January 1, 2027 (the "Remaining
                                                                       ---------
Life"), in each case, discounted to the prepayment date on a semi-annual basis
- ----                                                                          
(assuming a 360-day year consisting of twelve 30-day months) at the Adjusted
Treasury Rate, plus, in each case, accrued interest thereon to but excluding the
prepayment date.

          SECTION 3.02.  Notice of Prepayment.  Subject to Article Three of the
Base Indenture, notice of any prepayment pursuant to this Article 3 will be
mailed at least 30 days but not more than 60 days before the prepayment date to
each holder of Junior Subordinated Debentures to be prepaid at such holder's
registered address.  Unless the Company defaults in payment of the Event
Prepayment Price, on and after the prepayment date interest shall cease to
accrue on such Junior Subordinated Debentures called for prepayment.

          If the Trust is required to pay any additional taxes, duties or other
governmental charges as a result of a Tax Event, the Company will also pay any
Additional Tax Sums on the Junior Subordinated Debentures.


                                   ARTICLE 4

                      EXTENSION OF INTEREST PAYMENT PERIOD

          SECTION 4.01.  Extension of Interest Payment Period.  So long no Event
of Default under Section 6.01 of the Base Indenture has occurred and is
continuing, the Company shall have the right, subject to the provisions of
Section 2.10 of the Base Indenture, at any time during the term of the Junior
Subordinated Debentures, from time to time to defer the payment of interest by
extending the interest payment period of such Junior Subordinated Debentures for
a period not exceeding 10 consecutive semi-annual periods (an "Extension
                                                               ---------
Period"), during which Extension Period  the Company shall have the right to
make partial payments of interest on any Interest Payment Date.  No Extension
Period shall end on a date other than an Interest Payment Date or extend beyond
the Stated Maturity or any earlier prepayment date.  To the extent permitted by
applicable law, interest, the payment of which has been deferred because of an
Extension Period imposed pursuant to this Section 4.01, will bear Additional
Interest compounded semi-annually.  At the end of the Extension Period, the
Company shall pay all interest then accrued and unpaid on the Junior
Subordinated Debentures, including any Additional Interest, Special Interest and
Additional Tax Sums, if applicable, to the holders of the Junior 

                                      -10-

 
Subordinated Debentures in whose names the Junior Subordinated Debentures are
registered in the Register on the first Record Date preceding the end of the
Extension Period. Before the termination of any Extension Period, the Company
may further extend such Extension Period, provided that such period together
with all such further extensions thereof shall not exceed 10 consecutive semi-
annual periods, or extend beyond the Stated Maturity or any earlier prepayment
date. At any time following the termination of any Extension Period and upon the
payment of any accrued and unpaid Additional Interest and Special Interest then
due, the Company may elect to begin a new Extension Period, subject to the
foregoing requirements. No interest shall be due and payable during an Extension
Period, except at the end thereof.

          SECTION 4.02.  Notice of Extension.  (a)  If the Property Trustee is
the only registered holder of the Junior Subordinated Debentures at the time the
Company elects to begin or extend an Extension Period, the Company shall give
written notice to the Property Trustee and the Trustee of its election to begin
or extend any Extension Period at least five Business Days prior to the earlier
of (i) the next succeeding date on which Distributions on the Capital Securities
issued by the Trust would have been payable but for the election to begin or
extend such Extension Period or (ii) the date the Administrative Trustees are
required to give notice to any securities exchange or other applicable self-
regulatory organization or to holders of such Capital Securities of the record
date or the date such Distributions are payable, but in any event not less than
five Business Days prior to such record date. An Administrative Trustee shall
give notice of the Company's election to begin or extend an Extension Period to
the holders of such Capital Securities.

          (b)  If the Property Trustee is not the only holder of the Junior
Subordinated Debentures at the time the Company elects to begin or extend an
Extension Period, the Company shall give the holders of the Junior Subordinated
Debentures and the Trustee written notice of its election to begin or extend
such Extension Period at least ten Business Days prior to the earlier of (i) the
next succeeding Interest Payment Date or (ii) the date the Company is required
to give notice of the record or payment date of such interest payment to any
applicable self-regulatory organization or to holders of the Junior Subordinated
Debentures.

          (c)  The semi-annual period in which any notice is given pursuant to
paragraphs (a) or (b) of this Section 4.02 shall be counted as one of the 10
consecutive semi-annual periods permitted in the maximum Extension Period
permitted under Section 4.01.

 
                                   ARTICLE 5

                                   EXPENSES

      SECTION 5.01.  Payment of Expenses.  In connection with the offering, sale
and issuance of the Junior Subordinated Debentures to the Property Trustee and
in connection with the offering, sale and issuance of the Trust Securities by
the Trust, the Company, in its capacity as borrower with respect to the Junior
Subordinated Debentures, shall:

          (a)  pay all costs and expenses relating to the offering, sale and
     issuance of the Junior Subordinated Debentures, including commissions to
     the Purchasers payable 

                                      -11-

 
     pursuant to the Purchase Agreement and compensation of the Trustee under
     the Indenture in accordance with the provisions of Section 7.06 of the Base
     Indenture;

          (b)  pay all costs and expenses of the Trust (including, but not
     limited to, costs and expenses relating to the organization of the Trust,
     the fees and expenses of the Property Trustee and the Delaware Trustee, the
     costs and expenses relating to the operation of the Trust, including
     without limitation, costs and expenses of accountants, attorneys,
     statistical or bookkeeping services, expenses for printing and engraving
     and computing or accounting equipment, paying agent(s), registrar(s),
     transfer agent(s), duplicating, travel and telephone and other
     telecommunications expenses and costs and expenses incurred in connection
     with the acquisition, financing, and disposition of Trust assets);

          (c)  pay all costs and expenses related to the enforcement by the
     Property Trustee of the rights of the registered holders of the Capital
     Securities;

          (d)  be primarily liable for any indemnification obligations arising
     with respect to the Trust Agreement; and

          (e)  pay any and all taxes and all liabilities, costs and expenses
     with respect to such taxes of the Trust (but not including withholding
     taxes imposed on holders of Capital Securities or Common Securities of the
     Trust).

      SECTION 5.02.  Payment upon Resignation or Removal.  Upon termination of
this First Supplemental Indenture or the Base Indenture or the removal or
resignation of the Trustee pursuant to Section 7.10 of the Base Indenture, the
Company shall pay to the Trustee all amounts owed to it under Section 7.06 of
the Base Indenture accrued to the date of such termination, removal or
resignation.  Upon termination of the Trust Agreement or the removal or
resignation of the Delaware Trustee or the Property Trustee, as the case may be,
pursuant to Section 8.10 of the Trust Agreement, the Company shall pay to the
Delaware Trustee or the Property Trustee, and their respective counsel, as the
case may be, all amounts owed to them under Section 8.7 of the Trust Agreement
accrued to the date of such termination, removal or resignation.


                                   ARTICLE 6

                     FORM OF JUNIOR SUBORDINATED DEBENTURE

          SECTION 6.01.  Form of Junior Subordinated Debenture.  The Junior
Subordinated Debentures and the Trustee's Certificate of Authentication to be
endorsed thereon are to be substantially in the form attached hereto as Exhibit
A.

                                      -12-

 
                                   ARTICLE 7

               ORIGINAL ISSUE OF JUNIOR SUBORDINATED DEBENTURES

          SECTION 7.01.  Original Issue of Junior Subordinated Debentures.  (a)
Junior Subordinated Debentures in the aggregate principal amount of up to
$824,000,000 may be executed by the Company and delivered to the Trustee for
authentication by it, and the Trustee shall thereupon authenticate and deliver
said Junior Subordinated Debentures to or upon the written order of the Company,
signed by its Chairman of the Board, any Vice Chairman of the Board, the Chief
Executive Officer, the President, any Vice Chairman or any Vice President
(whether or not designated by a number or word or words added before or after
the title Vice President) and by its Treasurer, an Assistant Treasurer, the
Controller, its Secretary or an Assistant Secretary, without any further
corporate action by the Company as follows: (i) $824,000,000 aggregate principal
amount of Private Debentures to be originally issued on the Closing Date (as
defined in the Purchase Agreement) and (ii) $824,000,000 aggregate principal
amount of Exchange Debentures to be issued upon surrender of and in exchange for
the Private Debentures pursuant to Section 8.01.

          (b)  Each Exchange Debenture shall be issued only upon surrender of
and in exchange for a like aggregate principal amount of Private Debentures and
any Private Debentures surrendered in exchange for Exchange Debentures shall be
canceled. Accordingly, the aggregate principal amount of Private Debentures and
Exchange Debentures that may be outstanding at any time shall not exceed
$824,000,000.


                                   ARTICLE 8

                            EXCHANGE OF SECURITIES

          SECTION 8.01.  Mandatory Tender in Exchange Offer. The Junior
Subordinated Debentures will not be convertible into any other securities or
property of the Company.  The Junior Subordinated Debentures may not be
exchanged for Securities of any other series, except that if the Company effects
an exchange offer pursuant to Section 2(a) of the Debenture Exchange and
Registration Rights Agreement and, if pursuant to such exchange offer, the
Company offers to exchange any Private Debentures for Exchange Debentures, then,
to the extent permitted by law, each holder of the Private Debentures shall be
obligated to tender all the Private Debentures held by such holder in exchange
for a like principal amount of the Exchange Debentures in accordance with the
Company's instructions.


                                   ARTICLE 9

                                 MISCELLANEOUS

          SECTION 9.01.  Ratification of Indenture; First Supplemental Indenture
Controls.  The Indenture, as supplemented by this First Supplemental Indenture,
is in all respects ratified and confirmed, and this First Supplemental Indenture
shall be deemed part of the Indenture in the manner and to the 

                                      -13-

 
extent herein and therein provided. The provisions of this First Supplemental
Indenture shall supersede the provisions of the Indenture to the extent the
Indenture is inconsistent herewith.

          SECTION 9.02.  Trustee Not Responsible for Recitals.  The recitals
herein contained are made by the Company and not by the Trustee, and the Trustee
assumes no responsibility for the correctness thereof.  The Trustee makes no
representation as to the validity or sufficiency of this First Supplemental
Indenture.

          SECTION 9.03.  Governing Law.  This First Supplemental Indenture and
each Junior Subordinated Debenture shall be deemed to be a contract made under
the internal laws of the State of New York, and for all purposes shall be
governed by and construed in accordance with the laws of said State, without
regard to the conflicts of law principles thereof.

          SECTION 9.04.  Separability.  In case any one or more of the
provisions contained in this First Supplemental Indenture or in the Junior
Subordinated Debentures shall for any reason be held to be invalid, illegal or
unenforceable in any respect, such invalidity, illegality or unenforceability
shall not affect any other provisions of this First Supplemental Indenture or of
the Junior Subordinated Debentures, but this First Supplemental Indenture and
the Junior Subordinated Debentures shall be construed as if such invalid or
illegal or unenforceable provision had never been contained herein or therein.

          SECTION 9.05.  Counterparts.  This First Supplemental Indenture may be
executed in any number of counterparts each of which shall be an original; but
such counterparts shall together constitute but one and the same instrument.

                                      -14-

 
          IN WITNESS WHEREOF, the parties hereto have caused this First
Supplemental Indenture to be duly executed as of the date first above written.

                                    AON CORPORATION



                                    By:   /s/ Harvey N. Medvin
                                         -----------------------------------
                                          Name: Harvey N. Medvin
                                          Title: Executive Vice President, Chief
                                             Financial Officer and Treasurer



                                    By:   /s/ Raymond I. Skilling
                                         -------------------------------------
                                          Name: Raymond I. Skilling
                                          Title: Executive Vice President and
                                             Chief Counsel



                                    THE BANK OF NEW YORK,
                                    as Trustee


                                    By:   /s/ Mary La Gumina
                                         ------------------------------------
                                          Name: Mary La Gumina
                                          Title: Assistant Vice President

                                      -15-

 
                                                                       EXHIBIT A



               (FORM OF DEFINITIVE JUNIOR SUBORDINATED DEBENTURE)

          THE DEBENTURES EVIDENCED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
U.S. SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"), AND,
ACCORDINGLY, MAY NOT BE OFFERED, SOLD, OR OTHERWISE TRANSFERRED WITHIN THE
UNITED STATES OR TO, OR FOR THE ACCOUNT OR BENEFIT OF, U.S. PERSONS EXCEPT AS
SET FORTH IN THE FOLLOWING SENTENCE.  BY ITS ACQUISITION HEREOF, EACH HOLDER OF
THE DEBENTURES EVIDENCED HEREBY, AND EACH PERSON THAT ACQUIRES A BENEFICIAL
INTEREST IN  SUCH DEBENTURES, (1) REPRESENTS THAT (A) IT IS A "QUALIFIED
INSTITUTIONAL BUYER" (AS DEFINED IN RULE 144A UNDER THE SECURITIES ACT) OR (B)
IT IS AN INSTITUTIONAL "ACCREDITED INVESTOR" (AS DEFINED IN RULE 501
(a)(1),(2),(3) OR (7) UNDER THE SECURITIES ACT) ("INSTITUTIONAL ACCREDITED
INVESTOR") OR (C) IT IS NOT A U.S. PERSON AND IS ACQUIRING THE DEBENTURES
EVIDENCED HEREBY IN AN OFFSHORE TRANSACTION, (2) AGREES THAT IT WILL NOT, PRIOR
TO THE EXPIRATION OF THE HOLDING PERIOD APPLICABLE TO SALES OF THE DEBENTURES
EVIDENCED HEREBY UNDER RULE 144(k) UNDER THE SECURITIES ACT (OR ANY SUCCESSOR
PROVISION), OFFER, RESELL OR OTHERWISE TRANSFER THE DEBENTURES EVIDENCED HEREBY
EXCEPT (A) TO AON CORPORATION OR A SUBSIDIARY THEREOF, (B) PURSUANT TO AN
EFFECTIVE REGISTRATION STATEMENT UNDER THE SECURITIES ACT, (C) INSIDE THE UNITED
STATES TO A QUALIFIED INSTITUTIONAL BUYER IN COMPLIANCE WITH RULE 144A UNDER THE
SECURITIES ACT, (D) INSIDE THE UNITED STATES TO AN INSTITUTIONAL ACCREDITED
INVESTOR THAT, PRIOR TO SUCH TRANSFER, FURNISHES TO THE BANK OF NEW YORK, AS
PROPERTY TRUSTEE, A SIGNED LETTER CONTAINING CERTAIN REPRESENTATIONS AND
AGREEMENTS RELATING TO THE RESTRICTIONS ON TRANSFER OF THE DEBENTURES EVIDENCED
HEREBY (THE FORM OF WHICH LETTER CAN BE OBTAINED FROM SUCH TRUSTEE), (E) OUTSIDE
THE UNITED STATES IN COMPLIANCE WITH RULE 904 UNDER THE SECURITIES ACT OR (F)
PURSUANT TO AN EXEMPTION FROM REGISTRATION PROVIDED BY RULE 144 UNDER THE
SECURITIES ACT (IF AVAILABLE) AND (G) IN EACH CASE, IN ACCORDANCE WITH ANY
APPLICABLE SECURITIES LAWS OF THE STATES OF THE UNITED STATES OR ANY OTHER
APPLICABLE JURISDICTION, AND (3) AGREES THAT IT WILL DELIVER TO EACH PERSON TO
WHOM THE DEBENTURES EVIDENCED HEREBY IS TRANSFERRED A NOTICE SUBSTANTIALLY TO
THE EFFECT OF THIS LEGEND.  IF THE PROPOSED TRANSFEREE IS AN INSTITUTIONAL
ACCREDITED INVESTOR OR A PURCHASER WHO IS NOT A U.S. PERSON, THE HOLDER MUST,
PRIOR TO SUCH TRANSFER, FURNISH TO THE BANK OF NEW YORK, AS PROPERTY TRUSTEE,
SUCH CERTIFICATIONS OR OTHER INFORMATION AS AON CORPORATION MAY REASONABLY
REQUIRE TO CONFIRM THAT SUCH TRANSFER IS BEING MADE PURSUANT TO AN EXEMPTION
FROM, OR IN A TRANSACTION NOT SUBJECT TO, THE REGISTRATION REQUIREMENTS OF THE
SECURITIES ACT.  THIS LEGEND WILL BE REMOVED AFTER THE EXPIRATION OF THE HOLDING
PERIOD APPLICABLE TO SALES OF THE DEBENTURES EVIDENCED HEREBY UNDER RULE 144(k)
UNDER THE SECURITIES ACT OR SUCH EARLIER TIME AS A TRANSFER OF THE DEBENTURES
EVIDENCED HEREBY IS MADE PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER
THE SECURITIES ACT. 

 
AS USED HEREIN, THE TERMS "OFFSHORE TRANSACTION," "UNITED STATES" AND "U.S.
PERSON" HAVE THE MEANINGS GIVEN TO THEM BY REGULATION S UNDER THE SECURITIES
ACT.

          THE DEBENTURES EVIDENCED HEREBY WILL BE ISSUED, AND MAY BE
TRANSFERRED, ONLY IN BLOCKS HAVING A LIQUIDATION AMOUNT OF NOT LESS THAN
$100,000 (100 CAPITAL SECURITIES).  ANY TRANSFER, SALE OR OTHER DISPOSITION OF
SUCH DEBENTURES IN A BLOCK HAVING A LIQUIDATION AMOUNT OF LESS THAN $100,000
SHALL BE DEEMED TO BE VOID AND OF NO LEGAL EFFECT WHATSOEVER. ANY SUCH
TRANSFEREE SHALL BE DEEMED NOT TO BE THE HOLDER OF SUCH DEBENTURES FOR ANY
PURPOSE, INCLUDING BUT NOT LIMITED TO THE RECEIPT OF INTEREST IN RESPECT OF SUCH
DEBENTURES, AND SUCH TRANSFEREE SHALL BE DEEMED TO HAVE NO INTEREST WHATSOEVER
IN SUCH DEBENTURES.

 
                                   Registered
                                   ----------

FOR INFORMATION ON THE ISSUE PRICE, THE AMOUNT OF ORIGINAL ISSUE DISCOUNT, THE
ISSUE DATE AND THE YIELD TO MATURITY, PLEASE CONTACT AON CORPORATION, 123 NORTH
WACKER DRIVE, CHICAGO, ILLINOIS 60606, ATTENTION: TREASURER, TELEPHONE NO.:
312/701-3000.


NUMBER R-1                                              $824,000,000

                                AON CORPORATION
            8.205% JUNIOR SUBORDINATED DEFERRABLE INTEREST DEBENTURE
                              DUE JANUARY 1, 2027

Dated:  January 13, 1997                              [CUSIP__________________]


     Registered Holder:   The Bank of New York, as Property Trustee of Aon
Capital A

          AON CORPORATION, a corporation duly organized and existing under the
laws of the State of Delaware (herein referred to as the "Company," which term
includes any successor corporation under the Indenture hereinafter referred to),
for value received, hereby promises to pay to the Registered Holder named above,
the principal sum of Eight Hundred Twenty-Four Million Dollars ($824,000,000) on
January 1, 2027, in such coin or currency of the United States of America as at
the time of payment is legal tender for the payment of public and private debt.
The Company further promises to pay to the registered Holder hereof as
hereinafter provided (a) interest on said principal sum (subject to deferral as
set forth herein) at the rate per annum specified in the title of this debenture
(the "Debenture"), in like coin or currency, semiannually in arrears on the 1st
day of January and July (each an "Interest Payment Date") commencing July 1,
1997, from the Interest Payment Date next preceding the date hereof to which
interest has been paid or duly provided for (unless (i) no interest has yet been
paid or duly provided for on this Debenture, in which case from January 1, 1997,
or (ii) the date hereof is before an Interest Payment Date but after the related
Record Date (as defined below), in which case from such following Interest
Payment Date; provided, however, that if the Company shall default in payment of
the interest due on such following Interest Payment Date, then from the next
preceding Interest Payment Date to which interest has been paid or duly provided
for), until the principal hereof is paid or duly provided for, plus (b)
Additional Interest, as defined in the Indenture, to the extent permitted by
applicable law, on any interest payment that is not made on the applicable
Interest Payment Date, which shall accrue at the rate per annum specified in the
title of this Debenture, compounded semiannually. The interest so payable will,
subject to certain exceptions provided in the Indenture hereinafter referred to,
be paid to the person in whose name this Debenture is registered at the close of
business on the Record Date next preceding such Interest Payment Date.  The
Record Date shall be the Business Day next preceding the Interest Payment Date,
unless this Certificate is registered to a holder other than the Property
Trustee or a nominee of The Depository Trust Company, in which case the Record
Date will be the fifteenth day of the calendar month next preceding such
Interest Payment Date or, if such fifteenth day is not a Business Day, then the
Business Day next preceding such day.  This Debenture may be presented for
payment of principal and interest at the principal corporate trust office of The
Bank of New York, as paying agent for the Company, maintained for that purpose
in the Borough of Manhattan, The 

                                      A-1

 
City of New York, State of New York; provided, however, that payment of interest
may be made at the option of the Company (i) by check mailed to such address of
the person entitled thereto as the address shall appear on the Register of the
Debentures or (ii) by transfer to an account maintained by the Person entitled
thereto as specified in the Register, provided that proper transfer instructions
have been received by the Record Date. Interest on the Debenture will be
computed on the basis of a 360-day year of twelve 30-day months.
 
          So long as no Event of Default has occurred and is continuing, the
Company shall have the right at any time during the term of this Debenture from
time to time to defer payment of interest on this Debenture, for up to ten
consecutive semiannual interest payment periods with respect to each deferral
period (each an "Extension Period"), during which Extension Periods the Company
shall have the right to make partial payments of interest on any Interest
Payment Date; provided, however, that no Extension Period shall end on a date
other than an Interest Payment Date or extend beyond January 1, 2027 or any
earlier prepayment date.  At the end of each Extension Period, the Company shall
pay all interest then accrued and unpaid (together with any Additional Interest
thereon to the extent permitted by applicable law, Special Interest, and
Additional Tax Sums, if applicable).  During any such Extension Period, the
Company shall not, and shall cause any Subsidiary of the Company not to, (i)
declare or pay any dividends or distributions on, or redeem, purchase, acquire
or make a liquidation payment with respect to, any of the Company's Capital
Stock (which includes Common Stock and preferred stock) or (ii) make any payment
of principal of or interest or premium, if any, on or repay, repurchase or
redeem any debt securities of the Company that rank on a parity with or junior
to this Debenture or make any guarantee payments with respect to any Aon
Guarantee or other guarantee by the Company of the debt securities of any
Subsidiary of the Company that by its terms ranks on a parity with or junior to
this Debenture (other than (a) dividends or distributions in Common Stock, (b)
any declaration of a dividend in connection with the implementation of a Rights
Plan, the issuance of any Capital Stock or any class or series of preferred
stock of the Company under any Rights Plan or the redemption or repurchase of
any rights distributed pursuant to a Rights Plan, (c) payments under any Aon
Guarantee relating to the Preferred Securities issued by the Aon Trust holding
this Debenture, and (d) purchases of Common Stock related to the issuance of
Common Stock or rights under any of the Company's benefit plans for its
directors, officers, employees, consultants or advisors).  Prior to the
termination of any such Extension Period, the Company may further extend such
Extension Period; provided, however,  that no Extension Period shall exceed ten
consecutive semiannual periods or extend beyond January 1, 2027 or any earlier
prepayment date.  At any time following the termination of any Extension Period
and the payment of all accrued and unpaid interest (together with any Additional
Interest, Special Interest and Additional Tax Sums, if applicable) then due, the
Company may elect to begin a new Extension Period, subject to the above
requirements.  No interest shall be due and payable during an Extension Period
except at the end thereof.  If the Property Trustee is the only registered
holder of the Debentures of this series, the Company shall give written notice
to the Property Trustee and the Trustee of its election to begin or extend any
Extension Period at least five Business Days prior to the earlier of (i) the
next succeeding date on which Distributions on the Capital Securities issued by
the relevant Aon Trust would have been payable but for the election to begin or
extend such Extension Period or (ii) the date the Administrative Trustees are
required to give notice to any securities exchange or other applicable self-
regulatory organization or to holders of such Capital Securities of the record
date or the date such Distributions are payable, but in any event not less than
five Business Days prior to such record date.  An Administrative Trustee shall
give notice of the Company's election to begin or extend an Extension Period to
the holders of such Capital Securities.  If the Property Trustee is not the only
holder of the Debentures of this series at the time the Company elects to begin
or extend an Extension Period, the Company shall give the 

                                      A-4

 
holders of the Debentures of this series and the Trustee written notice of its
election to begin or extend such Extension Period at least ten Business Days
prior to the earlier of (i) the next succeeding Interest Payment Date or (ii)
the date the Company is required to give notice of the record or payment date of
such interest payment to any applicable self-regulatory organization or to
holders of the Debentures of this series.

          This Debenture is issued pursuant to an Indenture, dated as of January
13, 1997, between the Company, as issuer, and The Bank of New York, a New York
banking corporation, as trustee, as supplemented by a First Supplemental
Indenture dated as of January 13, 1997 (as further supplemented or amended from
time to time, the "Indenture").  Reference is made to the Indenture for a
description of the respective rights, limitations of rights, obligations, duties
and immunities thereunder of the Trustee, the Company and the Holders (the word
"Holder" or "Holders" meaning the registered holder or registered holders) of
the Debentures.  Capitalized terms used herein but not defined herein shall have
the respective meanings assigned thereto in the Indenture.  By acceptance of
this Debenture, the Holder hereof agrees to be bound by the provisions of the
Indenture.

          The Debentures of this series are limited to the aggregate principal
amount of Eight Hundred Twenty-Four Million Dollars ($824,000,000).

          The Debentures evidenced by this Certificate may be transferred or
exchanged only in minimum denominations of $100,000 and integral multiples of
$1,000 in excess thereof, and any attempted transfer, sale or other disposition
of Debentures in a denomination of less than $100,000 shall be deemed to be void
and of no legal effect whatsoever.

          The indebtedness of the Company evidenced by this Debenture, including
the principal hereof and interest hereon, is, to the extent and in the manner
set forth in the Indenture, subordinate and junior in right of payment to the
Company's obligations to Holders of Senior Indebtedness of the Company and each
Holder of this Debenture, by acceptance hereof, agrees to and shall be bound by
such provisions of the Indenture and all other provisions of the Indenture.

          This Debenture shall not be valid or become obligatory for any purpose
until the certificate of authentication hereon shall have been signed by or on
behalf of the Trustee under the Indenture.

                                      A-3

 
          IN WITNESS WHEREOF, AON CORPORATION has caused this instrument to be
signed, manually or in facsimile, by its Chairman of the Board, or its Chief
Executive Officer, or its President or any Vice Chairman, or any Vice President
and by its Treasurer or an Assistant Treasurer or its Controller or its
Secretary or an Assistant Secretary under the corporate seal of Aon Corporation.

                                    AON CORPORATION



                                    By:  ____________________________
                                          Name:
                                          Title:


                                    By:  ____________________________
                                          Name:
                                          Title:

[Seal]



                    TRUSTEE'S CERTIFICATE OF AUTHENTICATION

          This is one of the Securities, of the series designated herein,
described in the within-mentioned Indenture.

                                    THE BANK OF NEW YORK, as Trustee



                                    By:  ____________________________
                                         Authorized Signatory
                                         (by manual signature only)

                                      A-4

 
                             REVERSE OF DEBENTURE

          As provided in and subject to the provisions of the Indenture, if an
Event of Default with respect to the payment of principal or interest on the
Debentures of this series or with respect to compliance with certain covenants
occurs and is continuing, then either the Trustee or the Holders of not less
than 25% in principal amount of the then outstanding Debentures of each series
as to which such Event of Default has occurred may declare the principal amount
of all the Debentures of such series, together with any accrued interest
(including any Additional Interest, Special Interest and Additional Tax Sums),
to be due and payable immediately, by a notice in writing to the Company (and to
the Trustee, if such notice is given by Holders).  If the Debentures have been
issued to an Aon Trust, upon such an Event of Default, if the Trustee or the
Holders of not less than 25% in principal amount of the outstanding Debentures
of this series fails to declare the principal of all the Debentures to be
immediately due and payable, the holders of at least 25% in aggregate
liquidation amount of the corresponding Capital Securities of such Aon Trust
then outstanding shall have such right by a notice in writing to the Company and
the Trustee, and upon any such declaration the principal amount of and the
accrued interest (including any Additional Interest, Special Interest and
Additional Tax Sums) on all the Debentures of such series shall become
immediately due and payable, provided that the payment of principal and interest
on the Debentures shall remain subordinated to the extent provided in the
Indenture.

          If an Event of Default with respect to certain covenants applicable to
all series of securities issued under the Indenture (collectively, the
"Securities") occurs and is continuing, then either the Trustee or the Holders
of not less than 25% in principal amount of all then outstanding Securities
under the Indenture (voting as a single class) may declare the principal amount
of all such Securities to be due and payable immediately, by a notice in writing
to the Company (and to the Trustee if such notice is given by Holders).  If the
Securities of a series issued under the Indenture have been issued to an Aon
Trust, upon such an Event of Default, if the Trustee and the Holders of not less
than 25% in principal amount of all outstanding Securities of that series fail
to declare the principal of all the Securities of that series to be immediately
due and payable, the holders of at least 25% in aggregate liquidation amount of
the corresponding Capital Securities of such Aon Trust then outstanding shall
have such right by a notice in writing to the Company and the Trustee; and upon
any such declaration the principal amount of and the accrued interest (including
any Additional Interest and Special Interest) on all the Securities of that
series shall become immediately due and payable, provided that the payment of
principal and interest shall remain subordinated to the extent provided in the
Indenture.

          The Indenture provides that in certain events such declaration that
principal and accrued interest are due and payable, and the consequences of such
declaration, may be rescinded and annulled by the holders of at least a majority
in principal amount of the Securities then outstanding under the Indenture as to
which such acceleration of the payment of principal and interest has occurred,
voting as one class.  In the case of Securities issued under the Indenture to
one or more Aon Trusts, should the Holders of such Securities fail to rescind
and annul such declaration and its consequences, the Holders of at least a
majority in aggregate liquidation amount of the corresponding Capital Securities
of such Aon Trusts shall have such right.  The Indenture also provides that the
Holders of at least a majority in principal amount of all of the Securities of
all series then outstanding as to which an Event of Default has occurred may, on
behalf of all Holders of such Securities, waive any past default under the
Indenture other than (a) a default in the payment of the principal of or
interest on any of the Securities or (b) a default in respect of a covenant or
provision of the Indenture which under the terms of the Indenture cannot be
modified or amended without the consent of each Holder of Securities so
affected.  In the case of Securities of one or more series issued to one or more
Aon Trusts, the Indenture provides that the

                                      A-5

 
Holders of at least a majority in aggregate liquidation amount of the
corresponding Capital Securities issued by such Aon Trusts shall also have the
right to waive such defaults.

          The Indenture contains provisions permitting the Company and the
Trustee, with the consent of the Holders of not less than a majority in
principal amount of the outstanding Securities of all affected series (voting as
one class), to execute supplemental indentures adding any provisions to or
changing or eliminating any of the provisions of the Indenture or of any
supplemental indenture or modifying the rights of the holders of the Securities
of each such series; provided, however, that no such supplemental indenture
shall (i) change the fixed maturity of any Securities, or reduce the rate or
extend the time of payment of interest thereon or on any overdue principal
amount, or reduce the principal amount thereof, or reduce any amount payable
upon any redemption thereof, or make the principal thereof or any interest
thereon or on any overdue principal amount payable in any coin or currency other
than that herein prescribed, without the consent of the Holder of each security
so affected, (ii) reduce the percentage of Securities, the consent of the
Holders of which is required for any such supplemental indenture, without the
consent of all Holders of Securities then outstanding, (iii) modify certain
provisions of the Indenture relating to waiver of compliance with covenants,
waiver of defaults or modification of the Indenture without the consent of all
Holders of Securities then outstanding, except to increase the percentage of
Holders required for such waiver or modification, or (iv) modify the provisions
with respect to the subordination of outstanding Securities of any series in a
manner adverse to the Holders thereof, without the consent of the Holder of each
security so affected; provided, however, that, in the case of the Securities of
a series issued to an Aon Trust, so long as any of the corresponding series of
Capital Securities issued by such Aon Trust remains outstanding, (i) no such
amendment shall be made that adversely affects the holders of such Capital
Securities or Preferred Securities in any material respect, and no termination
of the Indenture shall occur, and no waiver of any Event of Default with respect
to such series or compliance with any covenant with respect to such series under
the Indenture shall be effective, without the prior consent of the holders of at
least a majority of the aggregate liquidation amount of such Capital Securities
then outstanding, unless and until the principal (and premium, if any) of the
Securities of such series and all accrued and unpaid interest (including any
Additional Interest) thereon shall have been paid in full and (ii) no amendment
shall be made to Section 6.05 of the Indenture (regarding the right of Holders
of Capital Securities to institute a suit directly against the Company) that
would impair the rights of the Holders of Capital Securities provided therein
without the prior consent of all Holders of Capital Securities then outstanding,
unless and until the principal (and premium, if any) of the Securities of such
series and all accrued and unpaid interest (including any Additional Interest)
thereon have been paid in full.

          Upon the occurrence and during the continuation of a Tax Event or
Investment Company Event, the Company may, at its option, at any time within 90
days of the occurrence of such Tax Event or Investment Company Event redeem this
Debenture in whole (but not in part) at a prepayment price (the "Event
Prepayment Price") equal to the greater of (i) 100% of the principal amount
hereof or (ii) as determined by a Quotation Agent, the sum of the present value
of 100% of the principal amount that would be payable on January 1, 2027,
together with the present values of scheduled payments of interest from the
prepayment date to January 1, 2027, in each case discounted to the prepayment
date on a semi-annual basis at the Adjusted Treasury Rate, plus, in each case,
accrued interest thereon to but excluding the date of prepayment.

          Any consent or waiver by the Holder of this Debenture given as
provided in the Indenture (unless effectively revoked as provided in the
Indenture) shall be conclusive and binding upon such Holder and upon all future
Holders of this Debenture and of any Debenture issued in exchange,

                                      A-6

 
registration of transfer, or otherwise in lieu hereof irrespective of whether
any notation of such consent or waiver is made upon this Debenture or such other
Debentures. No reference herein to the Indenture and no provision of this
Debenture or of the Indenture shall alter or impair the obligation of the
Company, which is absolute and unconditional, to pay the principal of and
interest on this Debenture, at the places, at the respective times, at the rate
and in the coin or currency herein prescribed.

          If the Company does not comply with certain of its obligations under
the Registration Rights Agreements (as defined in the Indenture), this Debenture
shall bear additional interest ("Special Interest") in addition to the interest
provided for in Section 2.05(a) of the First Supplemental Indenture to the
Indenture.

          As provided in the Indenture and subject to certain limitations
therein set forth, the transfer of this Debenture may be registered on the
Register of the Debentures of this series upon surrender of this Debenture for
registration of transfer at the offices maintained by the Company or its agent
for such purpose, duly endorsed by the Holder hereof or his attorney duly
authorized in writing, or accompanied by a written instrument of transfer in
form satisfactory to the Company and the Securities registrar duly executed by
the Holder hereof or his attorney duly authorized in writing, but without
payment of any charge other than a sum sufficient to reimburse the Company for
any tax or other governmental charge incident thereto.  Upon any such
registration of transfer, a new Debenture or Debentures of authorized
denomination or denominations for the same aggregate principal amount will be
issued to the transferee in exchange herefor.

          Prior to due presentment for registration of transfer of this
Debenture, the Company, the Trustee, and any agent of the Company or the Trustee
may deem and treat the person in whose name this Debenture shall be registered
upon the Register of the Debentures of this series as the absolute owner of this
Debenture (whether or not this Debenture shall be overdue and notwithstanding
any notation of ownership or other writing hereon) for the purpose of receiving
payment of or on account of the principal hereof and, subject to the provisions
on the face hereof, interest due hereon and for all other purposes; and neither
the Company nor the Trustee nor any such agent shall be affected by any notice
to the contrary.

          No recourse shall be had for the payment of the principal of or
interest on this Debenture, or for any claim based hereon or otherwise in
respect hereof, or based on or in respect of the Indenture or any indenture
supplemental thereto, against any stockholder, officer, director or employee, as
such, past, present or future, of the Company or of any successor corporation,
either directly or through the Company, whether by virtue of any constitution,
statute or rule of law, or by the enforcement of any assessment or penalty or
otherwise, all such liability being, by the acceptance hereof and as a part of
the consideration for the issue hereof, expressly waived and released.

          The Company and, by acceptance of this Debenture or a beneficial
interest in this Debenture, each holder hereof and any person acquiring a
beneficial interest herein, agree that for United States federal, state and
local tax purposes it is intended that this Debenture constitute indebtedness.

          This Debenture shall be deemed to be a contract made under the laws of
the State of New York (without regard to conflicts of laws principles thereof)
and for all purposes shall be governed by, and construed in accordance with, the
laws of said State.

                                      A-7

 
                                                                     EXHIBIT A-1

                        FORM OF CERTIFICATE OF TRANSFER


FOR VALUE RECEIVED, the undersigned hereby sells, assigns and transfers unto

Please insert Social Security or other
taxpayer identification number of transferee:

_______________________________________


________________________________________________________________________________
    (Name and Address of Transferee, including Zip Code, must be printed or
                                  typewritten)

________________________________________________________________________________

the within Junior Subordinated Deferrable Interest Debenture (the "Debenture")
and hereby irrevocably constitutes and appoints _________________________
attorney to transfer said Debenture on the Register of the Debentures, with full
power of substitution in the premises.

In connection with any transfer of the within Debenture occurring prior to such
date as restrictions on the transfer of such security imposed by the Securities
Act of 1933, as amended (the "Securities Act"), and the rules and regulations
thereunder shall be terminated in accordance with the Indenture, the undersigned
confirms that such Debenture is being transferred:

     [___]     To Aon Corporation or a subsidiary thereof; or
    

     [___]     Pursuant to an effective registration statement under the
               Securities Act; or


     [___]     Pursuant to and in compliance with Rule 144A under the Securities
               Act; or


     [___]     Pursuant to and in compliance with Regulation S under the
               Securities Act; or


     [___]     To an Institutional Accredited Investor pursuant to and in
               compliance with the Securities Act; or


     [___]     Pursuant to and in compliance with Rule 144 under the Securities
               Act:

and unless the box below is checked, the undersigned confirms that such
Debenture is not being transferred to an "affiliate" of Aon Corporation, as
defined in Rule 144 under the Securities Act (an "Affiliate"):

     [___]     The transferee is an Affiliate of Aon Corporation

                                      I-1

 
Date:  __________________________


                                    ___________________________________

                                    ___________________________________
                                              Signature(s)

                                    Signature(s) must be guaranteed.

          NOTICE:  The signature to this assignment must correspond with the
name as it appears upon the face of the within Junior Subordinated Deferrable
Interest Debenture in every particular, without alteration or enlargement or any
change whatever.

                                      I-2

 
                                                                       EXHIBIT B

                       FORM OF LETTER TO BE DELIVERED BY
                       INSTITUTIONAL ACCREDITED INVESTORS



The Bank of New York
101 Barclay Street
Floor 21 West
New York, New York  10286
Attention:  Corporate Trust Trustee
              Administration

Dear Sirs and Mesdames:

          We understand that the 8.205% Junior Subordinated Deferrable Interest
Debentures (the "Debentures") of Aon  Corporation ("Aon")  are being offered in
a transaction not involving any public offering within the United States within
the meaning of the Securities Act of 1933, as amended (the "Securities Act"),
and that the Debentures have not been registered under the Securities Act,  and
we agree, on our own behalf and on behalf of each account for which we acquire
any Debentures, that if, prior to the expiration of the holding period
applicable to sales of any Debenture under Rule  144(k) under the Securities
Act, we decide to offer, resell or otherwise transfer such Debenture, such
Debenture may be offered, resold or otherwise transferred only (i) to Aon or a
subsidiary thereof, (ii) pursuant to an effective registration statement under
the Securities Act, (iii) inside the United States to a person who is a
"qualified institutional buyer" (as defined in Rule 144A under the Securities
Act) in compliance with Rule 144A, (iv) inside the United States to an
Institutional Accredited Investor (as defined below) that, prior to such
transfer, furnishes to The Bank of New York, as trustee, a signed letter in the
form hereof and such other opinions and certifications that the Property Trustee
may request, (v) outside the United States in compliance with Rule 904 under the
Securities Act or (vi) pursuant to the exemption from registration provided by
Rule 144 under the Securities Act (if available) and (vii) in each case, in
accordance with any applicable securities laws of the States of the United
States or any other applicable jurisdiction and in accordance with the legends
set forth on the Debentures.  We further agree to provide any person purchasing
any of the Debentures from us a notice advising such purchaser that resales of
such securities are restricted as stated herein.  We understand that any
Debentures will be in the form of definitive physical certificates and that such
certificates will bear a legend reflecting the substance of this paragraph.

          We confirm that:

          (i) we are an "accredited investor" within the meaning of Rule
     501(a)(1), (2) or (3) under the Securities Act or an entity in which all of
     the equity owners are accredited investors within the meaning of Rule
     501(a)(1), (2) and (3) under the Securities Act (an "Institutional
     Accredited Investor");

                                      B-1

  
          (ii)   (A) any acquisition of Debentures by us will be for our own
     account or for the account of one or more other Institutional Accredited
     Investors or as fiduciary for the account of one or more trusts, each of
     which is an "accredited investor" within the meaning of Rule 501(a)(7)
     under the Securities Act and for each of which we exercise sole investment
     discretion or (B) we are a "bank," within the meaning of Section 3(a)(2) of
     the Securities Act, or a "savings and loan association" or other
     institution described in Section 3(a)(5)(A) of the Securities Act that is
     acquiring Debentures as fiduciary for the account of one or more
     institutions for which we exercise sole investment discretion;

          (iii)  in the event that we acquire any Debentures, we will acquire
     Debentures having a minimum purchase price of not less than $100,000 for
     our own account or for any separate account for which we are acting;

          (iv)   we have such knowledge and experience in financial and business
     matters that we are capable of evaluating the merits and risks of an
     investment in the Debentures;

          (v)    we are not acquiring Debentures with a view to resale or
     distribution thereof or with any present intention of offering or selling
     Debentures, except as permitted above; provided that the disposition of our
     property and property of any accounts for which we are acting as fiduciary
     shall remain at all times within our control; and

          (vi)   we have had access to such financial and other information and
     have been afforded the opportunity to ask such questions of representatives
     of Aon and receive answers thereto, as we deem necessary in connection with
     our decision to acquire Debentures.

          We acknowledge that Aon, you and others will rely upon our
confirmations, acknowledgments and agreements set forth herein, and we agree to
notify you promptly in writing if any of our representations or warranties
herein ceases to be accurate and complete.

          THIS LETTER SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH,
THE INTERNAL LAWS OF THE STATE OF NEW YORK.


                                    (Name of Transferee)



                                    By:  ____________________________________
                                          Name:
                                          Title:
                                          Address:

                                      B-2

 
                                                                       EXHIBIT C

                         FORM OF TRANSFER CERTIFICATE--
             144A GLOBAL DEBENTURE TO REGULATION S GLOBAL DEBENTURE



The Bank of New York
101 Barclay Street
Floor 21 West
New York, New York  10286
Attention:  Corporate Trust Trustee Administration

          Re:  Aon Corporation (the "Company") 8.205% Junior Subordinated
          Deferrable Interest Debentures due January 1, 2027 (the "Debentures")

          Reference is hereby made to the Indenture (the "Base Indenture") dated
as of January 13, 1997 between the Company and The Bank of New York, as Trustee
(as supplemented by the First Supplemental Indenture (the "Supplemental
Indenture") dated January 13, 1997, the "Indenture"). Capitalized terms used but
not defined herein shall have the meanings given to them in the Indenture.

          This letter relates to _________________ Private Debentures which are
evidenced by a 144A Global Debenture (CUSIP No. 37388AC9) and held with the
Depositary in the name of [insert name of transferor] (the "Transferor").  The
Transferor has requested a transfer of such beneficial interest in the Private
Debentures to a Person that will take delivery thereof in a transaction effected
pursuant to and in accordance with Rule 904 under the United States Securities
Act of 1933, as amended (the "Securities Act"), and accordingly the Transferor
does hereby further certify that:

          The offer of the Private Debentures was not made to a person in the
United States;

     (A)  either:

          (i)  at the time the buy order was originated, the transferee was
          outside the United States or the Transferor and any person acting on
          its behalf reasonably believed that the transferee was outside the
          United States, or

          (ii) the transaction was executed in, or through the facilities of a
          designated offshore securities market and neither the Transferor nor
          any person acting on its behalf knows that the transaction was pre-
          arranged with a buyer in the United States;

     (B)  no directed selling efforts have been made in contravention of the
          requirements of Rule 904(b) of Regulation S, as applicable;

                                      C-1

 
     (C)  the transaction is not part of a plan or scheme to evade the
          registration requirements of the Securities Act; and

     (D)  we have advised the transferee of the transfer restrictions
          applicable to the Private Debentures.

          You and the Company are entitled to rely upon this letter and are
irrevocably authorized to produce this letter or a copy hereof to any interested
party in any administrative or legal proceeding or official inquiry with respect
to the matters covered hereby.  Terms used in this certificate and not otherwise
defined herein or in the Indenture have the meanings set forth in Regulation S
under the Securities Act.

Dated:

                                    [Insert Name of Transferor]



                                    By:  ________________________________
                                          Name:
                                          Title:

(If the registered owner is a corporation, partnership or fiduciary, the title
of the Person signing on behalf of such registered owner must be stated.)

                                      C-2

 
                                                                       EXHIBIT D

                          FORM OF TRANSFER CERTIFICATE
             REGULATION S GLOBAL DEBENTURE TO 144A GLOBAL DEBENTURE


The Bank of New York
101 Barclay Street
Floor 21 West
New York, New York  10286
Attention:  Corporate Trust Trustee Administration

          Re:  Aon Corporation (the "Company") 8.205% Junior Subordinated
               Deferrable Interest Debentures due January 1, 2027 (the
               "Debentures")

          Reference is hereby made to the Indenture (the "Base Indenture") dated
as of January 13, 1997 between the Company and The Bank of New York, as Trustee
(as supplemented by the First Supplemental Indenture (the "Supplemental
Indenture") dated as of January 13, 1997, the "Indenture"). Capitalized terms
used but not defined herein shall have the meanings given to them in the
Indenture.

          This letter relates to _________________ Private Debentures which are
evidenced by a Regulation S Global Debenture (CUSIP No. U00199AB2) and held with
the Depositary indirectly in the name of [insert name of transferor] (the
"Transferor").  The Transferor has requested a transfer of such beneficial
interest in the Private Debentures to a Person that will take delivery thereof
in a transaction effected pursuant to and in accordance with Rule 144A under the
United States Securities Act of 1933, as amended (the "Securities Act"), and
accordingly the Transferor does hereby certify that the (i) the Transferor's
interest in the Private Debentures is being transferred in accordance with the
transfer restrictions set forth in the Indenture and in the Private Debenture;
and (ii) the transferee is a person who the Transferor reasonably believes is a
"qualified institutional buyer" within the meaning of Rule 144A under the
Securities Act, purchasing for its own account or the account of a qualified
institutional buyer in a transaction meeting the requirements of Rule 144A, in
accordance with all applicable securities laws of the states of the United
States and other jurisdictions.

          You and the Company are entitled to rely upon this letter and are
irrevocably authorized to produce this letter or a copy hereof to any interested
party in any administrative or legal proceeding or official inquiry with respect
to the matters covered hereby.

Dated:

                                    [Insert Name of Transferor]



                                    By:  ______________________________
                                          Name:
                                          Title:

(If the registered owner is a corporation, partnership or fiduciary, the title
of the Person signing on behalf of such registered owner must be stated.)

                                      D-1