A differential pulse polarographic (DPP) study of the Pb(2+)/Cys-Gly, Pb(2+)/gamma-Glu-Cys, Pb(2+)/PC(2) and Pb(2+)/PC(3) systems is performed, being PC(2) and PC(3) the phytochelatins of general structure (gamma-Glu-Cys)(n)-Gly, with n=2 and 3, respectively. Analysis of DPP data is assisted by multivariate curve resolution with alternating least squares (MCR-ALS) method in order to establish the complexes formation sequence and their final stoichiometries. DPP signals of these systems present, besides overlapping of peaks due to free metal ion and metal complexes, interference of mercury anodic signals. Despite these complications, MCR-ALS allows us to propose a model of complexation for each system, and some tentative structures for the complexes.