The epoxidation reactions of electron-deficient a,p-unsaturated carbonyl compounds were investigated in ionic liquid (IL)/water biphasic system using hydrogen peroxide as an oxidant at room temperature. By optimizing the reaction conditions, including reaction time, temperature, the amount of oxidant and sodium hydroxide, 100% conversion and 98% selectivity could be achieved in the epoxidation of mesityl oxide. The mass transfer model for the epoxidation of alpha,beta-unsaturated carbonyl compounds in the 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)MIm] [PF6])/water biphasic system has been proposed. (C) 2003 Elsevier Science B.V. All rights reserved.