The effect of tin in the depassivation of lead-calcium alloys under conditions of deep discharge (0.7 V versus Hg/Hg2SO4 in 0.5 M H2SO4) is investigated by measurements of the polarization resistance, the oxidation rate of ferrous ions added to the electrolyte, the potential decay with/without applied cathodic current, and surface analysis and visual inspection. The results may help to choose the optimal level of tin in the alloys.