Thick hydrous oxide films can be grown on Co metal electrodes in aqueous NaOH under conditions of potential cycling in cyclic voltammetry. Such films develop reversible reduction/reoxidation and corresponding pseudocapacitance behavior after formation by about 2800 cycles over the potential range -0.2 or +0.1 to 1.56 V at a sweep-rate of 20 mV s(-1). Equations expressing the origin of such pseudocapacitance, based on the Nernst equation for an electrochemical redox system, are derived and have similar forms to those for electrosorption pseudocapacitance. By recording voltammograms successively during a total history of 2800 cycles, the steady progression of behavior from completely irreversible oxide formation and reduction (like that at Pt or Au) to the development of a largely reversible redox oxide film is demonstrated. Progressive evaluation of the anodic and cathodic charges passed in each cycle, together with their charge-discharge imbalance, enables the total charge equivalent of the formed oxide to be calculated. This result shows that the anodic and cathodic charges which can be delivered reversibly after the thick film has been generated amount to only about 3% of the total formation charge. The reversible behavior of the developed film is therefore associated with an outer, near-surface region of the oxide, as has also been found with RuO2.