We apply a special form of ellipsometry, oblique incidence optical reflectivity difference (OI-RD), which is optimized to detect changes at a surface, to Co electrodeposition on polycrystalline Au. The OI-RD signal is shown to be proportional to Co thickness and thus enables the separation of the Co deposition and H-2 evolution partial currents. Competition between the two processes gives rise to interesting effects, including the observation of a peak in the Co partial current during both the anodic and cathodic sweeps of a cyclic voltammogram taken after repeated cycles of Co deposition and dissolution. (C) 2003 The Electrochemical Society.