Equilibrium study on the complex formation of M2+ ions (M = Co, Ni and Zn) with N-benzenesulphonyl-l-cysteine (NBSCH2) in the presence of bipyridine, ethylenediamine, glycinate (B) indicates the formation of mixed-ligand complexes of the types M(NBSC)(B), M(H-I NBSC)(B) and M(H-I NBSC)(OH), together with binary M(NBSC) and M(B) complexes. Formation constants of the complexes have been determined by potentiometric method at 25±0.1° in 50% (v/v) ethanol-water medium at a constant ionic strength, I=0.2 M (NaNO3). Stability of the complexes has been correlated with the modes of coordination of NBSC, basicity of the B ligands and π-electron delocalisation in the metal-ligands bonds.