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 Ni(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 SUPERQUAD computer program. In some systems Δ log K values are positive, i.e. the ternary complexes are found to be more stable than the corresponding binary complexes. H-bond formation seems to be most effective in deciding the stability of the ternary complexes formed in solution. Stabilities of mixed ligand complexes increases in the order AMP<ADP<ATP. With respect to the secondary ligands, the formation constants of the mixed lignads complexes decrease in the order succinic>maleic>tartaric>malic>citric>oxalic acid. © 1993 Springer-Verlag.