The epoxidation of alkenes with 30% aqueous hydrogen peroxide is catalyzed efficiently bymethyltrioxorhenium (MTO) in the presence of pyridine additives. The addition of 1−10 mol % of3-cyanopyridine increases the system's efficiency for terminal and trans-disubstituted alkenesresulting in high isolated yields of the corresponding epoxides. The system allows for epoxidationof alkenes with various functional groups. Alkenes leading to acid-sensitive products are efficientlyepoxidized using a mixture of pyridine and 3-cyanopyridine as additives. This method isoperationally very simple and uses an environmentally benign oxidant. The effects of differentpyridine additives on the alkene conversion and the catalyst lifetime are discussed.