Epoxides are an important class of industrial chemicals that have been used as chemical intermediates. Catalytic epoxidation of olefins affords an interesting production technology. We found a widely usable green route to the production of epoxides: A silicotungstate compound, [γ-SiW₁₀O₃₄(H₂O)₂]^4–, is synthesized by protonation of a divacant, lacunary, Keggin-type polyoxometalate of [γ-SiW₁₀O₃₆]^8–and exhibits high catalytic performance for the epoxidation of various olefins, including propylene, with a hydrogen peroxide (H₂O₂) oxidant at 305 kelvin. The effectiveness of this catalyst is evidenced by ≥99% selectivity to epoxide, ≥99% efficiency of H₂O₂utilization, high stereospecificity, and easy recovery of the catalyst from the homogeneous reaction mixture.