A catalytic amount of an oxalate/oxalic acid buffer strongly enhances the catalytic properties of Mn-tmtacn complexes for epoxidation reactions with H2O2. Especially terminal olefins are easily epoxidized. Yields for e.g. allyl acetate or 1-hexene reach up to 99% and 65% on olefin and peroxide basis respectively. The reaction is stereospecific; there are no products of solvolysis. (C) 1998 Elsevier Science Ltd. All rights reserved.