Oxidation reactions of alkenes with hydrogen peroxide were catalyzed by di-iron-substituted silicotungstate, gamma-SiW10{Fe(OH2)}(2) O-38(6-), with a high efficiency of hydrogen peroxide utilization. The efficiency of hydrogen peroxide utilization reached up to ca. 100% for the epoxidation of cyclooctene catalyzed by the di-iron-substituted silicotungstate. Not only cyclooctene but also 2-octene and cyclohexene were oxidized with a high efficiency of hydrogen peroxide utilization. The gamma-SiW10{Fe(OH2)}(2)O-38(6-) silicotungstate showed the highest efficiency of hydrogen peroxide utilization and conversion among di-transition-metal-and iron-substituted silicotungstates.