Cyclic voltammograms, current transients at constant potential and potential decay transients have been used to study the formation of lead dioxide surface films in the presence of cobalt ions and their role in decreasing the oxidation rate of a lead alloy under steady state conditions typical of copper electrowinning. The observations in the present work indicate, consistent with the surface film model, that the formation of a continuous PbSO4 + alpha-PbO2 film on the surface of the lead alloy in the presence of cobalt ions hinders further oxidation of the metal. The protectiveness of the film is dynamic in the steady state; the film is continuously forming and dissolving. Also studied was the potential of the oxygen evolution reaction on alpha-PbO2 and beta-PbO2 in 170 g L-1 H2SO4 with and without cobalt ions. The steady state potential for oxygen evolution on beta-PbO2 in 170 g L-1 H2SO4 at 285 A m(-2) decreased in the presence of cobalt ions and the steady state potential of beta-PbO2 was essentially the same as that of (i) the Pb-Ca-Sn alloy and (ii) alpha-PbO2. The implication is that the potential of the Pb-Ca-Sn alloy is determined by the alpha-PbO2 and/or beta-PbO2 on its surface.