The use of ion-selective electrodes for the measurement of the complexation properties of humic and fulvic acids is described. Values for mean molecular weight of the ligand, the stability constants of the complexes, number of ligands fixed per metal ion, and the dependency of the stability of the complexes on pH, may be obtained. The accuracy of the results is checked by comparing these results with those obtained by other methods and with known ligands. A comparison between this method and other existing methods for studying humic complexes is also made.