Potentiometric and EPR data allow for evaluation of the coordination equilibria in the Cu(2+)-bisphosphonate system. The bisphosphonic ligand was found very efficient in Cu(2+) chelation with formation of monomeric and dimeric species. Two phosphonate groups are basic binding sites for metal ion. The involvement of hydroxyl in metal ion coordination is also likely, especially when one phosphonate is protected by dimethyl ester. As the metal bound phosphonate groups are relatively bulky (six oxygens) and their negative charge above pH 4 is high (four per ligand) the equimolar species is a dominant complex at physiological pH.