The formation and growth of protective oxide films on nickel electrodes was studied in a pH 8.4 solution. A cathodic treatment of 10 min at −135V (vs. ) was sufficient for removal of the oxide film on electropolished nickel and thereby provided a starting point for the oxidation experiments. Anodization of the nickel electrode into the passive potential region produced a 9–12Aå oxide film on the surface, this being ca. 50% thicker than the oxide film on electropolished nickel. Although both types of oxide inhibited nickel dissolution, their cathodic electrochemical behavior was quite different, with the film on anodized nickel not being susceptible to cathodic reduction. Reflection electron diffraction showed that the lattice parameter of the oxide on anodized nickel was closer to stoichiometric than that on electropolished nickel. Formation of the oxide film on anodized nickel is by a direct electrochemical reaction, while that on electropolished nickel is probably due to the subsequent air exposure. The mechanism by which oxide films passivate nickel dissolution is discussed.