In situ observations of the mass change were carried out using the electrochemical quartz crystal microbalance (EQCM) technique for the active materials in a lead–acid battery during charge–discharge. Lead sulfate was formed on the surfaces of the pure Pb and Pb–Ca–Sn alloys immersed in 4.50kmolm−3 H2SO4 solution at 298K. The rates of PbSO4 formation on the Pb–0.08mass% Ca–Sn alloys, which are the choice materials for grids in the valve-regulated lead–acid battery (VRLA), were inhibited by the presence of Sn. This fact observed by the EQCM technique was in good agreement with the results determined by the prolonged corrosion test of 604.8ks at 348K. This state, in which the PbSO4 exists in the surface and the underlying Pb is not thoroughly reacted, corresponds to the state of active materials after discharge. During electrolytic oxidation, i.e. the charging of the positive electrode, the reaction of PbSO4→PbO2 could take place to decrease the electrode mass when the current density exceeded a critical value. On the other hand, the reaction of PbSO4→Pb could readily proceed at just one-fourth the current density of the electrolytic oxidation during the electrolytic reduction of the PbSO4/Pb electrode, i.e. the charging of the negative electrode.