The epoxidation of styrene using hydrogen peroxide and compressed CO2 was conducted in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-butyl-3-methylimidazolium hexafluoroborate ([bmim][PF6]), 1-butyl-3-methylimidazolium chloride ([bmim]Cl), 1,1,3,3-tetramethylguanidinium lactate [TMG][Lac], 1,1,3,3-tetramethylguanidinium trifluoroacetate [TMG][CF3COO], ethanol, and acetonitrile. It was demonstrated that the reaction could be carried out effectively in [bmim][BF4] or [bmim]Cl without any metallic catalysts. HCO4- produced from CO2 and H2O2 acted as the catalyst. The conversion of styrene and the yield of styrene oxide could reach 95% and 69%, respectively. (c) 2007 Elsevier B.V. All rights reserved.