The systems Cd2+-glycine and Pb2+-glycine, which form successive electrochemically labile complexes, were studied by several polarographic techniques: differential pulse, normal pulse and reverse pulse polarography. The voltammetric data were analysed by both the classical DeFord-Hume method (a hard modelling approach) and by a multivariate curve resolution method based on different factor analysis techniques (a soft modelling approach). The results have been compared with each other and also with those from the literature. It was found that while for labile complexes the necessary (for factor analysis techniques) condition of linearity of the current with respect to the concentrations is not always completely fulfilled, the results obtained by the two approaches are very similar. This suggests that for these systems the fraction of non-linearity of the signals is small and can be neglected. Thus, the proposed soft modelling approach also provided good estimations of the complexation parameters, and is suitable for use as a complementary tool on the study of systems with successive labile complexes.