The electrochemical oxidation and subsequent reduction of nickel electrodes in acid sulfate solutions was studied with potentiodynamic and galvanostatic techniques in conjunction with potential step polarization. Surface activity measurements gave a relative measure of the extent of surface oxidation and stability of the oxide toward reduction and dissolution, but did not give independently the oxygen coverage. Cathodic galvanostatic charging curves of previously anodized nickel electrodes displayed an arrest structure which was dependent on the extent of anodization. While the arrest charge was related to the cathodic reduction of some oxygen species on nickel, parallel side reactions such as nonelectrochemical dissolution and hydrogen evolution affected substantially the reduction current efficiency. Under certain conditions, the arrest charge component arising from cathodic reduction of the oxygen species on nickel could be resolved and an attempt made at identifying the nature of this species. Initially, anodization resulted in oxygen chemisorption which reached a maximum coverage corresponding to one oxygen atom per surface nickel. Conversion to nickel oxide occurred very rapidly at the active surface sites but more slowly over the rest of the surface.