Vanadyl salen complexes bearing electron-withdrawing substituents have been prepared and characterized. Systematic substitutions on the ancillary ligand have allowed V5+/V4+ 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 V5+/V4+ reduction potential. The mechanism likely involves oxygen transfer from intermediate hydroperoxides that are formed by radical-chain autoxidation.