A computer simulation of active-passive transition voltammograms of Pb in 5M H2SO4 is presented. The model implies alternative pathways leading to the formation of inner and outer structurally different PbSO4 layers. The inner layer consists of a disordered crystalline conducting phase, which is formed following an instantaneous nucleation and two-dimensional growth mechanism under charge-transfer control. Electrodissolution of Pb proceeds by the diffusion of cations through the inner layer and, when Ph(2+) ion concentration in solution exceeds the critical supersaturation concentration, the precipitation of the outer highly crystalline PbSO4 layer takes place, leading to passivation. Calculations are based on previously reported kinetic data. Good agreement was found between theory and practice.