Titanostannosilicalites molecular sieves as new compositions of matter have been found to be very selective, active catalysts in the epoxidation of olefins by peroxides. Dilute hydrogen peroxide suffices to afford high yields of the epoxide with good selectivity. The sieves described within have a unit empirical formula, on an anhydrous basis of (Ti.sub.x Sn.sub.y Si.sub.z)O.sub.2, where 0.0005