Interaction of adenosine-5′-triphosphate (ATP) with a series of binary Cu(II) complexes (ML) (where L =o-phenanthroline (Phen), 5-nitrophenanthroline (NPhen), 5-methyl phenanthroline (MPhen), 2,9-dimethylphenanthroline (DPhen), 2,9-dimethyl-4,7-diphenylphenanthroline (DPhPhen), 2,2′-bipyridyl (Bipy), bis(imidazol-2-yl) methane (BIM), oxalic acid (Ox), glycine (Gly), alanine (Ala), valine (Val), phenylalanine (Phe), tryptophan (Trp), methionine (Met), histidine (His) or aspartic acid (Asp) to form ternary complexes (MLA) was investigated by a pH-metric technique and the formation constants were evaluated at 35.0°C and μ = 0.2 M (KNO₃). The influence exerted by ligand L on the binding of ATP to Cu(II) was quantitatively assessed and ATP was found to bind more strongly when L = bidentate nitrogen donors, relatively less strongly when L = amino acids and least strongly when L = a bidentate oxygen donor. With respect to the nitrogen donor ligands the stability of the ternary complexes decreases in the order NPhen > Phen > Bipy > MPhen > DPhen > BIM ≈ DPhPhen, whereas in ternary complexes containing amino acids the stability decreases in the order: Phe ≈ Trp > Ala > Gly ≈ Val ≈ Met > His > Asp. The trends in the stability of the various complexes are discussed in terms of the nature of the metal ion and the two ligands in its coordination sphere.