The reduction process of tetra-PbO in different H2SO4 solutions has been studied by linear sweep voltammetry, potential step and scanning electron microscope (SEM) techniques. As the H2SO4 concentration decreases, more tetra-PbO is formed on a lead electrode and the PbSO4 crystals and the microvoid in the anodic film enlarge. This helps the mass transfer of the H+ and SO42- ions and the transformation of tetra-PbO into basic lead sulfates and PbSO4. The reduction mechanism of tetra-PbO changes from two- into three-dimensional progressive nucleation and growth of metallic lead with the decrease of the H2SO4 concentration. When PbSO4 begins to be reduced, tetra-PbO is transformed into PbSO4, which is then reduced.