Aon Announces Conversion Right Triggered for its 3 1/2% Senior Convertible Debentures.

Business Editors

CHICAGO--(BUSINESS WIRE)--April 1, 2004

Aon Corporation (NYSE:AOC) today announced that a conversion right has been triggered for its 3 1/2% Senior Convertible Debentures ($300 million) due November 15, 2012 based on the trading level of Aon's common stock. As provided in the indenture governing these convertible debentures, effective April 1, 2004, holders may exchange them for shares of Aon Corporation common stock at a conversion rate of 46.5658 shares per $1,000 principal amount of debentures, or a conversion price of $21.475. Holders may exercise the conversion right by contacting the Depository Trust Company and following the procedures set forth in the indenture in section 15.01.

Beginning second quarter 2004, common shares eligible to be received by the holders through the conversion (approximately 14 million) will be included in the dilutive average shares calculation regardless of whether the debentures are converted.

Aon Corporation (www.aon.com) is a holding company that is comprised of a family of insurance brokerage, consulting and insurance underwriting subsidiaries.

This press release contains certain statements relating to future results, which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results, depending on a variety of factors. Potential factors that could impact results include the general economic conditions in different countries around the world, fluctuations in global equity and fixed income markets, exchange rates, rating agency actions, pension funding, ultimate paid claims may be different from actuarial estimates and actuarial estimates may change over time, changes in commercial property and casualty markets and commercial premium rates, the competitive environment, the actual costs of resolution of contingent liabilities and other loss contingencies, the heightened level of potential errors and omissions liability arising from placements of complex policies and sophisticated reinsurance arrangements in an insurance market in which insurer reserves are under pressure, and the timing and resolution of related insurance and reinsurance issues relating to the events of September 11, 2001. Further information concerning the Company and its business, including factors that potentially could materially affect the Company's financial results, are contained in the Company's filings with the Securities and Exchange Commission.