The composition and morphology of the anodic film formed on a lead alloy under conditions typical of copper electro-winning was studied with the aim of establishing the mechanism by which cobalt ions decrease the oxidation rate of lead. The film was characterized by scanning electron microscopy, X-Ray Diffraction (XRD) and X-Ray Photon Electron Spectroscopy (XPS). The anodic film formed in the presence of cobalt ions was thin and compact whereas the film was thick and porous without cobalt ions. No crystalline cobalt compounds were detected by XRD and no cobalt peaks were detected by XPS. In the absence of cobalt ions in the solution, initial oxidation produces three layers on the lead surface, namely PbSO 4 , α-PbO 2 and β-PbO 2 in contact with the solution. In steady state, there are two layers: α-PbO 2 and β-PbO 2 . In the presence of a moderate cobalt ion concentration of up to and including 100 ppm, in steady state there remain the three layers: PbSO 4 , α-PbO 2 and β-PbO 2 . The results indicate that the presence of cobalt ions increases the imperviousness of the α-PbO 2 layer in hindering the oxidation of metallic lead to α-PbO 2 and also in hindering the oxidation of PbSO 4 to β-PbO 2 .