The influence of Sn on the formation of the corrosion layer on Pb has been studied with a Pb-10%Sn electrode in 0.5 and 4.5 M sulphuric acid using voltammetry, rrde and potential decay measurements, as well as photoelectrochemistry and ac impedance. The results obtained support the conclusion that in the presence of tin the amount of non-stoichiometric oxide formed in the corrosion layer in the vicinity of the equilibrium potential of the system PbO2/PbSO4 is increased. According to the results of the impedance measurements tin catalyses the oxidation of Pb(II) to Pb(IV) by 0.1-0.2V. It is suggested that the resulting, electronically conducting oxide is a mixed Pb-Sn oxide containing mainly Pb(IV).