Potentiometric equilibrium measurements have been made at 25.0±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 zwitterionic buffers (N,N-bis-(2-hydroxyethyl)-2-aminoethanesulphonic acid (BES), N-tris-(hydroxymethyl)-methyl-2-aminoethanesulphonic acid (TES), N,N-bis-(2-hydroxyethyl)-glycine (Bicine) and tris-(hydroxymethyl)-methylaminopropane sulphonic acid (TAPS)) 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 ligands have been refined with the SUPERQUAD computer program. Negative and positive Δ log K values were obtained for the ternary systems studied. 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. The trend in stability constants of the mixed-ligand complexes of the title zwitterionic buffer ligands is found to be TAPS>Bicine>TES>BES. © 1994 Springer-Verlag.