Protein-polyion films grown layer-by-layer and cast protein-surfactant films were employed on electrodes for catalytic oxidation of styrene derivatives to epoxides. Cytochrome P450cam and myoglobin in these films mediated the electrochemical reduction of oxygen to hydrogen peroxide, which activates these heme proteins to catalyze olefin oxidation. Compared to bare electrodes with the proteins dissolved in solution, ultrathin protein-polyion films on Au electrodes coated with mercaptopropane sulfonate gave the best catalytic activities for the oxidations. Improved performance of protein-polyion films is related to efficient, reversible heme Fe-III/Fe-II electron transfer and better mechanical stability than the surfactant films. Furthermore, dependence of product stereochemistry on oxygen availability in the reaction medium for the oxidation of cis-beta-methylstyrene suggested two pathways for olefin oxidation, which had not been reported previously for cyt P450 enzymes. The stereoselective pathway depends on an active, high-valent iron-oxygen intermediate as in the natural enzyme system, while the nonstereoselective pathway may involve a peroxyl radical near the protein surface.