Nitrobenzene (NB) including ferrocene was mixed vigorously with the sodium dodecylsulfate aqueous solution to yield an oil-in-water emulsion. Voltammograms of the emulsion showed an anodic wave at approximately 0.90 V versus Ag \ AgCl, which was discerned from the oxidation wave of micellized ferrocene in water (at 0.29 V), that of directly soluble ferrocene (at 0.22 V) and also that of ferrocene in NB (at 0.55 V). From dependence of the anodic current on the scan rate, the anodic wave was due to the adsorption on oil droplets including ferrocene. In contrast, a cathodic wave was only the reduction of the soluble ferrocinium ion at 0.15 V. Consequently, ferrocene, which is known as an electrochemically reversible couple, behaves as an irreversible system in the emulsion. When a potential of more than 1.2 V was applied, the emulsion was gradually separated into the oil and the aqueous phases, regardless of the presence or absence of ferrocene. A possible reaction mechanism is adsorption of oil droplets containing ferrocene at the electrode, an increase in the interfacial energy with the application of the positive potential, coalescence of the droplets, dispersion of ferrocene in the water phase, and the oxidation of the ferrocene at the electrode. An optical microscopic view showed spheres of oil less than 7 mum in diameter. The emulsions were stable for a few weeks without agitation.