Solution equilibrium studies on the M(II) [Co(II), Cu(II), and Ni(II)] N,N'-(S,S)-bis[1-carboxy-2-(imidazol-4-yl)ethyl]ethylenediamine (BCIEE) systems have been performed by pH potentiometry and UV-visible spectroscopy in 0.1 M KCl at 25 degrees C. Overall stability constants were established, by potentiometry, for all M(II)-BCIEE systems. Results evidenced the formation of a highly stable ML complex with all M(II)-BCIEE systems, which is the dominant species over a wide range of pH, plus different protonated, MHxL, complexes. From potentiometric and spectroscopic results, it was shown that, for ML complexes, BCIEE binds to Co(II) or Cu(II) through the two amine and two imidazole groups in the chelate plane and through the carboxyl groups in the apical positions, while the coordination of the NiL complex is assumed to be by four nitrogen groups in the plane. In the case of MHL complexes in the Cu(II)-BCIEE and Ni(II)-BCIEE systems, BCIEE behaves as a tridentate ligand [via COO-, NH2, and N(Im) groups] coordinating to the metal ion in the chelate plane. Results also suggest that BCIEE behaves as a bidentate ligand [via COO- and N(Im) or via NH2 and N(Im) groups], forming an equilibrium mixture of two CuH2L structures for the Cu(II)-BCIEE system.