Potentiometric equilibrium measurements have been made at (25 ± 0.1) °C and ionic strength I = 0.1 mol dm−3 KNO3 for the interaction of adenosine 5′-mono-, 5′-di-, and 5′-triphosphate (AMP, ADP, and ATP) and Cu(II) with biologically important secondary ligand acids (malic, maleic, succinic, tartaric, and oxalic acids) in a 1:1:1 ratio and the formation of various 1:1:1 mixed ligand complex species inferred from the Potentiometric pH titration curves. Initial estimates of the formation constants of the resulting species and the acid dissociation constants of AMP, ADP, ATP, and secondary ligand acid have been refined with the SUPERQUAD computer program. Δ log K values are positive; i.e., the ternary complexes are found to be more stable than the corresponding binary complexes. In some Cu(II) ternary systems studied the interligand interactions or some cooperativity between the coordinate ligands, possibly H bond formation, has been found to be most effective in deciding the stability of the ternary complexes formed in solution. Stabilities of mixed ligand complexes increase in the order AMP < ADP < ATP. With respect to the secondary ligands, the formation constants of the mixed ligand complexes decrease in the following order: succinic > malic > maleic > tartaric > oxalic. © 1993, American Chemical Society. All rights reserved.