Further evidence that the rate-controlling process in self-discharge of the nickel oxide electrode is the anodic partial reaction of oxygen evolution is reported and is based on: (a) comparison of the heats of activation for open-circuit oxygen evolution and for d-c. anodic polarization with oxygen evolution; (b) comparison of the current potential behavior for d-c. anodic polarization and for rates of oxygen evolution on open circuit as a function of potential; and (c) comparison of H/D isotope effects for open-circuit and d-c. polarization behavior. In the latter cases, an unusual and characteristic inverse isotope effect is observed.True Tafel slopes are deduced and interpreted in terms of possible mechanisms of oxygen evolution, taking account of the dependence of activation energy upon surface coverage by adsorbed intermediates.