Abstract: The stability constants of ternary complexes of Cd(ATP)(Aa)and of corresponding binary complexes have been determined by potentiometric pH titration in solution at 37℃ and at 0.10 mol/L ionic strength(ATP=adenosine triphosphate,Aa=alanine(ala),methionine(met),phenylalanine(phe)or tryptophan(trp)respectively).The relative stabilities of the ternary complexes in the amino acid ligand order are trp^- >met^- >phe^- >ala^-.The possible reasons have been discussed in terms of intramolecular ligand-ligand interactions specially the hydrophobic association including aromatic ring stacking.In order to characterize the intramolecular ligand-ligand interaction quantitatively,the equilibrium constants,K₁,of transfer between opened species(in the absense of the ligand-ligand interaction)and closed species(with ligand-ligand interaction)have been calculated.The results show that the closed species of ternary complexes containing trp,met or phe as one of ligands are the major components in solution.