Olefin epoxidation by Mo(VI) peroxo complexes was computationally investigated (DFT B3LYP) for various monoperoxo models X2MoO(O-2)(H2O)(NH3) with anionic ligands X and for the experimentally known complex MoO(O-2) (dipic)(H2O). All these monoperoxo complexes exhibit higher barriers for direct oxygen transfer to ethylene than the reference bisperoxo complex MoO(O-2)(2)(H2O)(NH3) with the same base ligand configuration; the most electronegative ligands X induce the lowest barriers. A molecular orbital analysis reveals factors that govern the activity of the peroxo ligand and corroborates the electrophilic character of the attack of a peroxo group on the olefin.