Potentiometrie 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’-triphosphates (AMP, ADP, and ATP) and Co(II) with biologically important secondary ligand zwitterionic buffers N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES), N-[tris(hydroxymethyl)methyl]-2-aminoethanesulfonic acid (TES), N,N-bis(2-hydroxyethyl)glycine (Bicine), and N-[tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid (TAPS) in a 1:1:1: ratio and the formation of various 1:1:1 mixed ligand complex species inferred from the Potentiometrie pH titration curves. Initial estimates of the formation constants of the resulting species and the acid dissociation 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. In some Co(II) ternary systems the interligand interactions 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. The trend in stability constants of the mixed-ligand complexes of the zwitterionic buffer ligands is in the order TAPS > Bicine > TES > BES. © 1995, American Chemical Society. All rights reserved.