Some basic elements are developed for the theory of the voltammetric response of a heterogeneous metal complex system, i.e. a system in which the effective stability of the complex species varies with the metal-to-ligand ratio. The treatment accounts for (1) a Freundlich-type binding characteristic, (2) differences between the diffusion coefficients of complexed and uncomplexed metal species, and (3) the kinetics of the complex association/dissociation reactions, but it is limited to cases with (i) an overall excess of ligands, (ii) a non-variant association rate constant, so that the stability distribution is completely reflected by the dissociation rate constant distribution. Lability criteria are derived and separated into homogeneous and heterogeneous factors. Various aspects are further explained and illustrated by some numerically simulated polarographic waves.