The epoxidation of cyclohexene to cyclohexene oxide by dioxiranes in the ketone/oxone system was studied. The catalytic activities of acetone, butan-2-one, pentan-2-one and 1,1,1-trifluoroacetone were investigated. A monophasic and a biphasic system were investigated for this reaction, namely tetrahydrofuran (THF)/water and dichloromethane/water. The THF/water system was found to be more efficient at converting cyclohexene to cyclohexene oxide. It was found that the rate of cyclohexene oxide formation increased in proportion to increasing ketone concentration using 1,1,1-trifluoroacetone, acetone and butan-2-one. Turnover frequencies were in the range 0-2.75 h(-1), in sharp contrast to the perceived reactivity of this system. The greater the solubility of a ketone in water the greater its catalytic activity. The selectivity in terms of cyclohexene conversion to cyclohexene oxide was always 100% but the selectivity in terms of KHSO5 utilisation was generally less than 60%, even in the best experimental conditions. (C) 1998 SCI.