A chiral gold(III) complex has been prepared that performs the epoxidation of olefins in the presence of O-2, PhIO, or bleach. Catalytic experiments with O-18 show that O-2 is activated on the catalyst and can be directly incorporated into the epoxide through a non-radical mechanism that probably involves formation of gold, oxo, or peroxo species. In addition to this, there is a parallel radical mechanism operating that yields alpha, beta-unsaturated ketones and alcohols as subproducts. Electrochemical and UV-Vis experiments confirmed the occurrence of a Au(III)/Au(I) redox cycle during the catalytic epoxidation in a mechanism sustained by molecular oxygen. (C) 2009 Elsevier Inc. All rights reserved.