The cyclohexene epoxidation by molecular oxygen in the presence of tetraphenylporphyrin chloride of Mn, Fe, Co and Ru and isobutyraldehyde have been investigated. The results showed that high yield of epoxide was obtained by the liquid phase epoxidation of cyclohexene in the presence of isobutyraldehyde using Mn(TPP)Cl as catalyst. The reaction mechanism has been proposed, and the results indicated (according to the mechanism) that cyclohexene epoxide was mainly formed from the reaction of high-valent metal oxo intermediate with olefin directly, which was verified by in situ EPR and in situ UV-vis spectroscopy respectively. The factors influencing epoxide generation rate, e.g. the concentration of cyclohexene, manganese porphyrin catalyst and isobutyraldehyde, have been well investigated. The kinetics of cyclohexene aerobic epoxidation developed from the proposed mechanism was consistent with the experimental kinetics accurately. (C) 2009 Elsevier B.V. All rights reserved.