The planned relocation of Aon to London could see company founder Patrick Ryan facing a multi-million dollar capital gains tax (CGT) bill, reports Crain's. Aon's January 13 filing with the SEC states that for US tax purposes a US investor in Aon would recognize a gain "equal to the excess fair market value of the Aon UK shares ... over such holder's adjusted basis in the Aon Delaware shares exchanged". The US has a 15% long-term capital gains tax rate and Mr Ryan (with his wife) has a 4% stake – 12.8m shares – in Aon. Depending on the basis price for Mr Ryan's stock, his tax bill could be anywhere from $30m to $80m. The report claimed that Mr Ryan was said to be upset by the tax situation. Individual investors who hold Aon stock in their retirement plans will not be subject to CGT. Some of Mr Ryan's stock is also understood to be held in a trust for beneficiaries, which will complicate the CGT situation. Although Aon CEO Greg Case will also see a CGT bill on his holding of around 450,000 shares, Morningstar analyst Theo Francis noted that Mr Case stands to gain more than half a million dollars a year additional compensation for relocating to London.