A process for preparing an epoxidized .alpha.-olefin/diene copolymer including the steps of contacting in a reaction medium a) copolymers of an .alpha.-olefin and a diene, which copolymers contain at least one double bond in each diene-derived co-monomer, with b) a hydrogen peroxide epoxidizing agent, in the presence of a perfluorinated alcohol reaction solvent, and in the absence of any metal-containing epoxidation catalyst, under reaction conditions which promote formation of oxirane rings at the sites of the diene-derived co-monomer double bonds in the copolymeric precursor material.