The evolution of hydrogen acid oxygen was studied on diamond electrodes containing approximately 10(21) boron atom/cm(3). Voltammetry showed a wide potential window [-1.25 to +2.3 V vs. standard hydrogen electrode (SHE)] without significant water decomposition. This window was much narrower for poor quality diamond films with appreciable sp(2) content. A redox couple observed at +1.7 V indicates oxidation of the diamond surface prior to oxygen evolution. The extent of surface oxidation increased with sp(2) content. Anodic polarization made the diamond surface hydrophilic; x-ray photoelectron spectroscopy showed an increase in oxygen coverage and the presence of carbon-oxygen bonds. The estimated capacitance of the interface ranged from 0.05 mu F/cm(2) for high quality diamond to 5 mu F/cm(2) for low quality diamond. Preliminary measurements of the exchange current densities for oxygen and hydrogen evolution indicated slow kinetics compared to metals or highly oriented pyrolytic graphite.