The formation of metal cation complexes between o-phenylenediamine with metal ions, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺ and Cr³⁺ were studied in the dimethylformamide/water(DMF/H₂O), acetonitrile/water(AN/H₂O) and ethanol/water(EtOH/H₂O) binary systems using square wave polarography (SWP) technique. The stoichiometry and stability of the complexes were determined by monitoring the shifts in half-waves or peak potentials of the polarographic waves of metal ions against the ligand concentration. In the most cases, the formation constants of complexes decreased with increasing amounts of H₂O, DMF and EtOH in AN/H₂O, DMF/H₂O and EtOH/H₂O binary systems, respectively. The stoichiometry of the complexes was found 1:1. The results obtained show that there is an inverse relationship between the formation constant of the complexes and the donor number of the solvents based on the Gatmann donocity scale. Also, the stability constants show a high sensitivity to the composition of the mixed solvent systems. In most of the systems investigated, Cr³⁺ cation forms a more stable complex with o-phenylenediamine than other four cations and the order of selectivity of this ligand for cations in pure water is:Cr³⁺>>Cu²⁺>Ni²⁺>Zn²⁺>Pb²⁺