A fundamental study is undertaken of the mechanism of the formation and dissolution of lead sulfate on the negative electrode in a lead–acid battery. This involves in situ examination of a lead sheet, on which a small amount of BaSO4 or SrSO4 powder is fixed by pressing, in sulfuric acid solution by means of electrochemical atomic force microscopy (EC-AFM) combined with cyclic voltammetry (CV). It is found that both BaSO4 and SrSO4 provide seed crystals for the precipitation of PbSO4. The PbSO4 crystals are formed more rapidly on SrSO4 than on BaSO4 during the oxidation (discharge) process. The dissolution of PbSO4 crystals formed on SrSO4 crystals is very slow during the reduction (charge) process.