The process of formation of a passivating sulphatic film on electrodes made of lead-tin alloys was studied by voltammetry. This process was established to proceed under diffusive control and to be limited to mass transfer in the anode film. At anodic dissolution of lead-tin alloys, there proceeds active dissolution of tin which collects inside the anode film and passes to solution. Owing to the formation of a more porous sulphatic film on the surface of a tin-containing alloy, the constant of diffusive process increases.