Equilibrium studies on the mixed-ligand complex formation of M2+ ions (M = Co, Ni, Cu and Zn) with sulfapyridine, sulfadiazine and sulfathiazole (LH) in the presence of glycine, β-alanine and dl-methionine (AH) in 50% (v/v) ethanol-water medium, I = 0.1 M (NaNO3) at 30 ± 1° have provided evidence of complexes M(L)(A), M(L)(A)(OH)- and M(L)(A)(OH)22-. Stability constants of the complexes are determined and complex formation equilibria correlated with the modes of coordination by the ligands.