Voltammetric metal speciation in mixtures of inert and labile complexes is discussed, especially for the case of the titrations of ligand mixtures with metal ion solutions, frequently used in determining complexing capacities. A new voltammetric model is applied which, for the case of non-excess of ligands forming inert complexes, produces a set of equations that can be used to obtain complexation parameters of both labile and inert complexes from titration data. The applicability of these equations is tested by simulation and by fitting them to the experimental results obtained by differential pulse polarography for the system Zn(II) + nitrilotriacetate + polymethacrylate in the presence of succinate buffer. The study showed that the method provides an accurate measure of the total concentration of the 'inert' ligand, and a rough estimate of the stability constants of both inert and labile complexes. It was also noticed that, when macromolecular labile complexes are present, the complexation constant thus determined can be a meaningless parameter unless such complexes are considered.