The iron heme protein myoglobin (Mb) coadsorbs with surfactant onto glassy carbon, pyrolytic graphite, and platinum electrodes from microemulsions of sodium. dodecyl sulfate or cetyltrimethylammonium bromide, water, oil, and pentanol. Adsorbates on all electrodes gave reversible heme Fe-III/Fe-II voltammetry for Mb, which spectroscopy showed to be in near-native conformation. The nature of the adsorbed films was highly dependent on the electrode material. The carbon electrodes gave nonideal thin-film voltammetry, but Pt electrode voltammetry was more consistent with thick films featuring in-film diffusion of protein. Mb films on all electrodes supported electrochemical catalytic reduction of 1,2-dibromocyclohexane (DBCH) and trichloroacetic acid (TCA). Results on carbon electrodes were consistent with Michaelis-Menten enzyme kinetics. The catalytic rate and binding of reactant to Mb were more efficient for DBCH in microemulsions than in aqueous buffer but were more efficient for TCA in aqueous buffer. Microemulsions exerted a tuning effect on the reactivity of these reactants according to their hydrophobicity. The more hydrophobic DBCH had better access to Mb in the surfactant films and reacted at a faster rate, while the hydrophilic TCA had poor access to the catalyst in the film.