A theoretical study of the effect of the physico-chemical heterogeneity of complexants such as fulvic compounds on d.c. polarographic curves is described. These compounds have been strongly implicated in playing a decisive role in trace cation complexation in natural systems. Their main feature is their wide range of complexation energy, which stems from their intrinsic properties: polyfunctionality, polyelectrolytic character and conformational properties. The whole of these complexing properties may be represented by complexation distribution functions. It is shown that, in the formation of labile metal—fulvic complexes and in the absence of adsorption and kinetic effects, it is possible to obtain from a single voltammetric curve important parameters related to metal speciation, namely the average diffusion coefficient of the complexes and the parameters describing the equilibrium complexation distribution functions.