Methyltrioxorhenium (MTO)-catalyzed epoxidation of alkenes with H₂O₂ has been significantly improved by using 3-methylpyrazole as an additive. A system consisting of 35% H₂O₂ and MTO–3-methylpyrazole in CH₂Cl₂ catalyzes the epoxidation of various alkenes in excellent yields. The catalytic activity of MTO–3-methylpyrazole surpasses MTO–pyrazole and MTO–pyridine catalysts. Quantitative yields of epoxides from cyclic and internal alkenes were obtained with only 0.05–0.1 mol% of MTO in the presence of 10 mol% of 3-methylpyrazole.