A set of experiments on the complexation of cadmium and lead with humic acids were performed at pH 5.0 using normal- and reverse-pulse polarography and d.c. and differential-pulse anodic stripping voltammetry. The concentration of deprotonated groups at pH 7.0, the average stability parameter, ($) over bar K, the differential equilibrium function, f(K-DEF), and a parameter, Gamma, proportional to the metal buffer capacity of the system and related to the heterogeneity of the ligand, were calculated. It was concluded that the complexes are labile within the experimental error for the time scale of the techniques; that lead complexes are more stable than cadmium complexes and that the ligand has a more heterogeneous behaviour with respect to lead than to cadmium