The heterogeneity of natural organic ligands in the presence of a metal ion affects its concentration profile in the diffusion layer. In fact, owing to heterogeneity, the ratio of complexed metal to free metal increases in the diffusion layer with proximity to the electrode surface. The diffusing species are then decelerated because the diffusion coefficient of the macromolecular complex is lower than that of the metal ion, and so the total amount of reduced metal is decreased compared with that obtained in a homogeneous system with the same bulk speciation. Therefore, the voltammetric current, which is directly related to the concentration profile in the diffusion layer, is affected by heterogeneity. In the work reported in this paper the concentration profile in the diffusion layer was determined on the basis of the diffusion process and a heterogeneity model describing the concentration of the different complexing sites and their complexing affinity for the metal. The mean stability parameters for the complexation of natural organic ligands with a metal ion were determined from this profile, simulating the value that should be determined from voltammetric currents. The difference between the simulated value and the mean stability parameter related to bulk speciation was analysed.