Potentiometric equilibrium measurements have been made at 25±0.1°C (μ=0.1 mol dm-3 KNO3) for the interaction of adenosine-5′-mono-, -di- and -triphosphate (AMP, ADP and ATP) and Cu(II) with biologically important secondary ligand acids (malic, maleic, succinic, tartaric, citric 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. In some systems Δlog 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 the interligand interactions between the coordinate ligands, possibly H bond formation, have 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 order succinic > maleic > tartaric > malic > citric > oxalic acid. © 1993 Springer-Verlag.