During charge and discharge of lead-acid batteries the concentration of the H2SO4 solution in the pores of the active material and in the interface grid/active mass varies widely. In this investigation, the influence of pH of the H2SO4 solution on the phase composition of the positive active mass (PAM) and of the interface PAM/grid is studied. The influence of pH on the phase composition of the interface is determined indirectly by cycling Pb electrodes between 0.70 and 1.60 V (versus Hg/Hg2SO4) in H2SO4 solutions of various concentrations and determining the phase composition and the structure of the anodic layer formed. The influence of pH on the phase composition of the PAM is investigated by immersing fully charged PAM samples into H2SO4 solutions of various concentrations and determining the phase composition of the PAM and the size of the alpha-PbO2, beta-PbO2 and PbSO4 crystals. It has been found that the outer sub-layer of the anodic layer participates in the cycling processes and its phase composition depends on the pH of the solution and on the potential scan rate. The reduction rate of PbO2 in this sub-layer depends on the solution pH. If the reduction of PbO2 proceeds in solutions with pH between -1.0 and -0.50, the rate of the processes is high. When it proceeds at pH > -0.50 the reduction rate is lower. This behaviour of the PbO2/PbSO4 electrode influences the power performance of the lead-acid battery when the positive plates are the power limiting component. The rate of oxidation of PbSO4 to PbO2 is determined by the pH of the solution because the solubility of PbSO4 depends on pH. In concentrated solutions the solubility of PbSO4 is low, the charge process is slow and some unoxidised PbSO4 may remain in the charged plate. In diluted H2SO4 solutions, the solubility of PbSO4 is high and PbSO4 crystals are oxidised fully during charge. It has been found that the phase composition of the PAM depends on the pH of the solution since the hydrated part of the PbO, particles interacts with the ions in the solution as a result of which the crystal zones/hydrated zones and hydrated zones/solution equilibria are changed. The content of alpha-and beta-PbO2 crystal zones in the PAM depends on the pH of the solution. In the pH region between -0.75 and 0, SO42- ions which have penetrated into the hydrated gel zones react with Ph2+ ions from the nonstoichiometric part of the PAM (PbO2-delta) forming PbSO4 molecules, which leave the hydrated zones and enter the solution forming PbSO4 crystals there. When the PAM is immersed in solutions with pH between 0 and -1.0, the content of the crystal phases of alpha- and beta-PbO2 decreases. In this pH region the average size of the beta-PbO2 crystals decreases while that of alpha-PbO2 remains constant or increases slightly. The results of this investigation evidence that the PAM is a dynamic system, which interacts with the ions in the solution, and hence the phase composition of the PAM depends on the concentration of the H2SO4 Solution.