The mixed-ligand complex formation in the systems Hg2+ - Edta(4-) - L-(L=Arg, Orn, Ser) has been studied by means of calorimetry, pH-potentiometry and NMR spectroscopy in aqueous solution at 298.15 K and the ionic strength of I=0.5 (KNO3). The thermodynamic parameters of formation of the HgEdtaL, HgEdtaHL and (HgEdta)(2)L complexes have been determined. The most probable coordination mode for the complexone and the amino acid in the mixed-ligand complexes was discussed. (C) 2012 Elsevier B.V. All rights reserved.