Equilibrium study on the mixed-ligand complex formation of the 3d(M II) ions (M = Co, Ni, Cu and Zn) with glycine (H3N +CH2COO-, GlyH±) and 4,6-dimethyl-2-guanidinopyrimidine [H2NC(=NH)NH{CN 2(CCH3)2CH}, Gup] indicate variety of binary and mixed-ligand complexes: (M(Gly)+], [M(Gly)(OH)], [M(Gup) 2+], [M(Gup-H)+]. [M(GuP)22+], [M(Gup-H)(Gup)+], [M(Gup-H)2], [M(Gly)(Gup)+] and [M(Gly)(Gup-H)]. With CuII and NiII, the mixed-ligand complexes [M(Gly)(Gup)+] and [M(Gly)(Gup-H)] dominate the pH range of biological relevance, whereas, with CoII and ZnII the [M(Gly)+] and [M(Gly)(OH)] complexes are the major metal containing species under the same condition. Formation constants of the proton-ligand and metal-ligand complexes at 25 ± 1 °C in 50% v/v ethanol-water medium (ionic strength, I = 0.1 mol dm-3) were evaluated by computer based pH-metric titration method. Complex formation equilibria were elucidated by analyzing the speciation curves and correlated with the modes of coordination of the ligands.