Ferrylmyoglobin species, which are active oxidant forms of the protein myoglobin, were obtained by electrochemical reduction of metmyoglobin [MbFe(III)] in the presence of oxygen in aqueous neutral buffer and in microemulsions of oil, water, and cationic surfactant. Reduction of myoglobin at -0.4 V vs SCE catalyzed the reduction of oxygen to hydrogen peroxide at the electrode. Hydrogen peroxide oxidizes metmyoglobin in solution to give the radical ferrylmyoglobin . X-MbFe(IV)=O, which is known to decay rapidly to the non-radical MbFe(IV)=O. This complex reduction-oxidation process converted nearly all of the 30 mu M myoglobin in a spectroelectrochemical cell to ferrylmyoglobins in 15 min in pH 7.3 buffer and in 18 min in microemulsions. Characteristic ferryl heme absorbance bands near 421, 548, and 584 nm were used to identify products. Confirmation of ferrylmyoglobins was provided by reductions to metmyoglobin with ascorbate and by myoglobin-mediated electrochemical epoxidation of styrene. Fiftyfold higher yields of styrene oxide and benzaldehyde were achieved in a microemulsion compared to electrochemical or chemical methods in pH 7.4 buffer. The electrochemical approach described may also prove useful for investigating active catalytic species of heme enzymes such as cytochrome P450.