Vanadyl salen complexes bearing electron-withdrawing substituents have been prepared and characterized. Systematic substitutions on the ancillary ligand have allowed V(5+)/V(4+) reduction potentials to be tuned over a range of approximately 500 mV. The complexes are catalysts for the aerobic epoxidation of cyclohexene; catalytic activity roughly increases with increasing V(5+)/V(4+) reduction potential. The mechanism likely involves oxygen transfer from intermediate hydroperoxides that are formed by radical-chain autoxidation.