The oxo-imido molybdenum complex 2a is an effective catalyst at low catalyst loadings (0.5 mol% or below) for the epoxidation of a range of alkenes with tBuOOH in PhMe at 90° C. Reactions are complete in less than 4h and the products are isolated in high yields. The catalytic system is chemoselective for the epoxidation of electron-rich alkenes and allylic alcohols.