NOTICE OF GUARANTEED DELIVERY
                                 for Tender of
                           8.205% Capital Securities
               (Liquidation Amount $1,000 per Capital Security)
                                      of
                                 Aon Capital A


     As set forth in the Exchange Offer (as defined below), this Notice of
Guaranteed Delivery (or a facsimile hereof) or one substantially equivalent
hereto or the electronic form used by The Depository Trust Company ("DTC") for
this purpose must be used to accept the Exchange Offer if certificates for
8.205% Capital Securities due January 1, 2027 (the "Old Capital Securities") of
Aon Capital A, a Delaware statutory business trust (the "Trust"), are not
immediately available to the registered holder of such Old Capital Securities,
or if a participant in DTC is unable to complete the procedures for book-entry
transfer on a timely basis of Old Capital Securities to the account maintained
by The Bank of New York (the "Exchange Agent") at DTC, or if time will not
permit all documents required by the Exchange Offer to reach the Exchange Agent
prior to 5:00 p.m., New York City time, on ________________, 1997, unless
extended (the "Expiration Date"). This Notice of Guaranteed Delivery (or a
facsimile hereof) or one substantially equivalent hereto or the electronic form
used by DTC may be delivered by mail (registered or certified mail is
recommended), by facsimile transmission, by hand or overnight carrier to the
Exchange Agent. See "The Exchange Offer -Procedures for Tendering Old Capital
Securities." Capitalized terms used herein and not defined herein have the
meanings assigned to them in the Exchange Offer.

                            The Exchange Agent is:

                             The Bank of New York


                                                                
By Registered or Certified Mail:             By Facsimile:              By Hand/Overnight Carrier:
  The Bank of New York                    The Bank of New York             The Bank of New York
  101 Barclay Street - 7E                 Attn:  Enrique Lopez              101 Barclay Street
New York, New York 10286                     (212) 571-3080           Corporate Trust Services Window
Attn:  Reorganization Section                                                  Ground Level
                                    (For Eligible Institutions Only)     New York, New York  10286
                                         Confirm by Telephone:         Attn:  Reorganization Section
                                            (212) 815-6333
 
                                         For Information Call:
                                            (212) 815-6333


     Delivery of this Notice of Guaranteed Delivery to an address other than as
set forth above or transmission of this Notice of Guaranteed Delivery via a
facsimile number other than the number listed above will not constitute a valid
delivery.

     This Notice of Guaranteed Delivery is not to be used to guarantee
signatures. If a signature on a Letter of Transmittal is required to be
guaranteed by an Eligible Institution (as defined therein) under the
instructions thereto, such signature guarantee must appear in the applicable
space provided in the signature box on the Letter of Transmittal.



Ladies and Gentlemen:

     The undersigned hereby tenders to the Trust and to Aon Corporation, a
Delaware corporation ("Aon"), the aggregate liquidation amount of Old Capital
Securities indicated below pursuant to the guaranteed delivery procedures and
upon the terms and subject to the conditions set forth in the accompanying
Prospectus dated__________, 1997 (as the same may be amended or supplemented
from time to time, the "Prospectus") and in the related Letter of Transmittal
(which together with the Prospectus constitute the "Exchange Offer"), receipt of
which is hereby acknowledged.

     The undersigned hereby represents, warrants and agrees that the undersigned
has full power and authority to tender, exchange, sell, assign, and transfer the
Tendered Old Capital Securities and that the Trust will acquire good, marketable
and unencumbered title thereto, free and clear of all liens, restrictions,
charges and encumbrances when the Tendered Old Capital Securities are acquired
by the Trust as contemplated herein, and the Tendered Old Capital Securities are
not subject to any adverse claims or proxies. The undersigned warrants and
agrees that the undersigned and each Beneficial Owner will, upon request,
execute and deliver any additional documents deemed by Aon, the Trust or the
Exchange Agent to be necessary or desirable to complete the tender, exchange,
sale, assignment and transfer of the Tendered Old Capital Securities, and that
the undersigned will comply with its obligations under the Registration Rights
Agreement. The undersigned has read and agrees to all of the terms of the
Exchange Offer.

     BY TENDERING OLD CAPITAL SECURITIES AND EXECUTING THIS NOTICE OF GUARANTEED
DELIVERY, THE UNDERSIGNED HEREBY REPRESENTS AND WARRANTS THAT (i) NEITHER THE
UNDERSIGNED NOR ANY BENEFICIAL OWNER(S) IS AN "AFFILIATE" OF AON OR THE TRUST,
(ii) ANY NEW CAPITAL SECURITIES TO BE RECEIVED BY THE UNDERSIGNED AND ANY
BENEFICIAL OWNER(S) ARE BEING ACQUIRED BY THE UNDERSIGNED AND ANY BENEFICIAL
OWNER(S) IN THE ORDINARY COURSE OF BUSINESS OF THE UNDERSIGNED AND ANY
BENEFICIAL OWNER(S), (iii) THE UNDERSIGNED AND EACH BENEFICIAL OWNER HAVE NO
ARRANGEMENT OR UNDERSTANDING WITH ANY PERSON TO PARTICIPATE IN A DISTRIBUTION
(WITHIN THE MEANING OF THE SECURITIES ACT) OF NEW CAPITAL SECURITIES TO BE
RECEIVED IN THE EXCHANGE OFFER, AND (iv) THE UNDERSIGNED OR ANY SUCH BENEFICIAL
OWNER IS NOT ENGAGED IN, AND DOES NOT INTEND TO ENGAGE IN, A DISTRIBUTION
(WITHIN THE MEANING OF THE SECURITIES ACT) OF SUCH NEW CAPITAL SECURITIES. 

     A broker-dealer who holds Old Capital Securities for its own account as a
result of market-making activities or other trading activities and who receives
New Capital Securities in exchange for such Old Capital Securities pursuant to
the Exchange Offer may be deemed to be an "underwriter" within the meaning of
the Securities Act and will be required to deliver a prospectus meeting the
requirements of the Securities Act in connection with any resale of such New
Capital Securities. If the undersigned or any beneficial owner(s) is a broker-
dealer which acquired any of the Tendered Old Capital Securities for its own
account as the result of market-making activities or other trading activities,
such broker-dealer acknowledges that it will deliver a prospectus meeting the
requirements of the Securities Act in connection with any resale of New Capital
Securities received in exchange for any of such Tendered Old Capital Securities
that were acquired for its own account as the result of market-making activities
or other trading activities (provided that, by so acknowledging and by
delivering a prospectus, such broker-dealer will not be deemed to admit that it
is an "underwriter" within the meaning of the Securities Act).

     All questions as to the form of documents, validity, eligibility (including
time of receipt) and acceptance for exchange of Tendered Old Capital Securities
will be determined by Aon and the Trust, in their sole discretion, whose
determination shall be final and binding on all parties. Aon and the Trust
reserve the absolute right, in their sole and absolute discretion, to reject any
and all tenders determined by Aon and the Trust not to be in proper form or the
acceptance of which, or exchange for, may, in the view of Aon and the Trust or
of counsel to Aon and the Trust, be unlawful.

     All authority herein conferred or agreed to be conferred shall survive the
death or incapacity of the undersigned and every obligation of the undersigned
hereunder shall be binding upon the heirs, executors, administrators, personal
representatives, trustees in bankruptcy, legal representatives, successors and
assigns of the undersigned.

                                      -2-


Name(s) of Registered Holder(s):
                                ------------------------------------------------

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                                 Please Print

Address(es):
            --------------------------------------------------------------------


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

Area Code and Tel. No(s):
                         -------------------------------------------------------


     X
      --------------------------------------------------------------------------

     X
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               Signature(s) of Owner(s) or Authorized Signatory


     Must be signed by the registered holder(s) of the Tendered Old Capital
Securities as their name(s) appear(s) on certificates for such Tendered Old
Capital Securities, or on a security position listing, or by person(s)
authorized to become registered holder(s) by endorsement and documents
transmitted with this Notice of Guaranteed Delivery. If signature is by a
trustee, executor, administrator, guardian, attorney-in-fact, officer or other
person acting in a fiduciary or representative capacity, such person must set
forth his or her full title below.



                           Aggregate Liquidation
 Certificate No(s)          Amount Represented          Aggregate Liquidation
   (if available)             by Certificate               Amount Tendered
- -------------------      -------------------------      ---------------------
                                                  
       
- -------------------      -------------------------      ---------------------
 
- -------------------      -------------------------      ---------------------

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

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

If Old Capital Securities will be delivered by book-entry transfer to The
Depository Trust Company, provide the following information:

Signature:
          ------------------------------------------------------------
Account Number:
               -------------------------------------------------------
Date:
     -----------------------------------------------------------------



              THE GUARANTEE ON THE REVERSE SIDE MUST BE COMPLETED


                                      -3-


                                   GUARANTEE
                    (Not to be used for signature guarantee)

     The undersigned, a firm or other entity identified in Rule 17Ad-15 under
the Securities Exchange Act of 1934, as amended, as an "eligible guarantor
institution," including (as such terms are defined therein):  (i) a bank; (ii) a
broker, dealer, municipal securities broker, municipal securities dealer,
government securities broker, government securities dealer; (iii) a credit
union; (iv) a national securities exchange, registered securities association or
clearing agency; or (v) a savings association that is a participant in a
Securities Transfer Association recognized program (each of the foregoing being
referred to as an "Eligible Institution"), hereby guarantees delivery to the
Exchange Agent, at one of its addresses set forth above, either certificates for
the Old Capital Securities tendered hereby, in proper form for transfer, or
confirmation of the book-entry transfer of such Old Capital Securities to the
Exchange Agent's account at The Depository Trust Company ("DTC"), pursuant to
the procedures for book-entry transfer set forth in the Prospectus, in either
case together with one or more properly completed and duly executed Letter(s) of
Transmittal (or facsimile thereof or an Agent's Message in lieu thereof) and any
other documents required by the Letter of Transmittal, all within five (5)
business days after the date of execution of this Notice of Guaranteed Delivery.

     The undersigned acknowledges that it must communicate the guarantee to the
Exchange Agent and must deliver the Letter of Transmittal and certificates for
the Old Capital Securities tendered hereby to the Exchange Agent within the time
period shown hereon and that failure to do so could result in a financial loss
to the undersigned.

_________________________________     __________________________________________
              Firm                               Authorized Signature


_________________________________     Name______________________________________
             Address                              (Please Type or Print)


_________________________________     Title_____________________________________
            Zip Code

                                      Dated_______________________________, 1996

Area Code and Tel. No.:_________________________________________________________

     DO NOT SEND CERTIFICATES FOR OLD CAPITAL SECURITIES WITH THIS NOTICE OF
GUARANTEED DELIVERY.  ACTUAL SURRENDER OF OLD CAPITAL SECURITIES MUST BE MADE
PURSUANT TO, AND BE ACCOMPANIED BY, A PROPERLY COMPLETED AND DULY EXECUTED
LETTER OF TRANSMITTAL AND ANY OTHER REQUIRED DOCUMENTS.


                                      -4-