The anodic dissolution of Ni, Si, and Sb and of the compounds , , , and was studied in molar perchlorate solutions at pH 0.04 and 10.8. In all cases, except with Si in acid solution, active dissolution at low anodic potentials is followed by a transition to a passive state where the anodic current density is nearly independent of potential. In acid solution, the critical potentials of , , and are less positive than that of Ni and the critical currents are less by two to three orders of magnitude. The critical potential of is about the same as for Sb, but the critical current is substantially less. The anodic current in the passive region for the compounds is greater than for passive Ni. , , and show a transpassive region in which Sb, As, and S are probably oxidized to ions of a high valence state. In alkaline solution, , , and show an active‐passive transition at the same potential as pure Ni. However, the rate of oxidation of these compounds in the passive state is again considerably greater than for passive Ni. It is suggested that in acid solution the passive film on nickel compounds is either a mixed oxide or an oxide having the basic structure of the second element; in alkaline solution, nickel oxide is formed first, but it probably has a different structure than the oxide formed on passive Ni. The difference between acid and alkaline solutions arises from the difference in the stability of nickel oxide in the two electrolytes. A stable, passive oxide on Ni is formed in acid solution only when the dissolution rate is sufficiently large to cause nickel hydroxide to precipitate at the electrode surface, a condition not fulfilled during the dissolution of any of the compounds. In general, the oxide of the companion element or a mixed oxide is more stable than nickel oxide in acid solution at low potentials. This situation is reversed in alkaline solution, and in this case the initial passive film is nickel oxide.The electrocatalytic reduction of oxygen on nickel and nickel compounds occurs in the potential region where a passive film is stable. The order of catalytic activity for oxygen reduction is in acid solution, and in alkaline solution.