In order to elucidate the growth mechanism of the passive alpha-PbO at the interface grid/positive active material of the lead-acid batteries, a sequential electrochemical method using O-18 was set up. Tt was established that the formation of a thick, dense PbO layer on pure lead and lead alloys (Pb-Sb, Pb-Ca-Sn), in potential conditions simulating deep discharged batteries for electric vehicles, is the consequence of a solid-state process, with O2- as the predominant diffusing species. Diffusion of O2- anions occurs along the grain boundaries of PbO, as well as in the grain bulk, probably between the two neighboring planes of Pb2+ cations of the PbO tetragonal structure.