A density functional study on olefin epoxidation by rhenium and molybdenum peroxo complexes has been carried out. Various intermediates and transition structures of the systems CH3ReO3/H2O2, H3NMoO3/H2O2, and H3NOMoO3/H2O2 were characterized, including ligated and unligated mono- and bisperoxo intermediates as well as hydroperoxo derivatives. For the rhenium system the bisperoxo complex CH3ReO(O2)2*H2O was found to be most stable and the one with the lowest transition state for epoxidation of ethylene (activation barrier of 16.2 kcal/mol), in line with experimental findings. However, participation of monoperoxo and hydroperoxo complexes in olefin epoxidation cannot be excluded. For both molybdenum systems, hydroperoxo species with an additional ammonia model ligand in axial position were calculated to be most stable. Inspection of calculated activation barriers of ethylene epoxidation reveals that, in both molybdenum systems, hydroperoxo mechanisms are competitive if not superior to peroxo mechanisms. The reaction barriers of the various oxygen transfer processes can be rationalized by structural, orbital, and charge characteristics, exploiting a model that interprets the electrophilic nature of the reactive oxygen center.