The mechanism of [γ-H2SiV2W10O 40]4--catalyzed epoxidation of alkenes with hydrogen peroxide in acetonitrile/tert-butyl alcohol was investigated. The negative Hammett ρ+ (-0.88) for the competitive oxidation of p-substituted styrenes and the low XSO (XSO = (nucleophilic oxidation)/(total oxidation)) value of <0.01 for the [γ-H 2SiV2W10O40]4--catalyzed oxidation of thianthrene-5-oxide reveal that the strong electrophilic oxidant species is formed on [γ-H2SiV2W10O 40]4- (I). The preferable formation of trans-epoxide for the epoxidation of 3-substituted cyclohexenes shows the steric constraints of the active oxidant on I. The 51V NMR, 183W NMR, and CSI-MS spectroscopy show that the reaction of I with hydrogen peroxide leads to the reversible formation of a hydroperoxo species [γ-HSiV2W 10O39OOH]4- (II). The successive dehydration of II forms III, which possibly has an active oxygen species of a μ-η2:η2-peroxo group. The kinetic and spectroscopic studies show that the present epoxidation proceeds via III. The energy diagram of the epoxidation with density functional theory (DFT) supports the idea.