The stability constants of the ternary Cu 2+ complexes containing adenosine 5′-monophosphate (AMP) and as a second ligand, 2,2′-bipyridyl, 4- (2′-pyridyl) imidazole, 2-picolylamine, 4-aminomethylimidazole, and ethylenediamine were determined by potentiometric titration in water containing 10% dioxane (I=0.1; 25°C). For the equilibrium, Cu (amine) 2+ +Cu (AMP) ⇄Cu (amine) (AMP) +Cu 2+ , the corresponding constants are (in the order of the above amine ligands), Δ log K =log = 0.53 , 0.20 , 0.04 , —0.35 , and —0.45 ; hence , in the first cases the equilibrium is considerably on the side of the mixed-ligand complex. The importance of the π-system of the amine for the stability of the mixed-ligand Cu 2+ complexes is obvious from the given series. The imidazole group, an important binding site for metal ions in biological systems, has clearly qualities similar to the pyridyl group (Inorg. Chem. 9, 1238 ), i. e. the stability of ternary Cu 2+ complexes containing these groups is increased.