A mechanism has been established by further studying the growth of the lead sulfate layer. At very small overpotentials, lead sulfate crystals are formed through solution-precipitation mechanism.(1-2) However, as the anodic overpotential increases, the migration and diffusion of the SO42- and Pb2+ ions through the micropores of the anodic film become the controlling step. At short times, the flow of the SO42- ion is dominant and its concentration increases linearly with the distance from the metal surface. But at long times, the flow of the Pb2+ ion is dominant and its concentration drops almost linearly with the distance from the metal surface. Wherever the concentration of the dominant ion is small, the gradient of the diffusion potential is large. It is also shown that the grown current of the lead sulfate layer drops linearly with t(-0.5) and there are two slopes to this curve which corresponds to the processes above.