Thermodynamic parameters of etidronic acid [(1-Hydroxy-1,1-ethanediyl) bis(phosphonic acid), HEDPA] interaction with Hg2+ and CH3Hg+ were determined in NaCl aqueous solution at different ionic strength values (0.1 <= I <= 1 mol.L-1) and at T = 298.15 K. For Hg2+ system, ML2-, MLH02, MLH3+ and ML(OH)(3-) species were obtained. In presence of CH3Hg+, ML3-, MLH2-, MLH2-, MLH30 and the dinuclear M2LH- and M2LH20 species were determined with formation constant values lower respect to the Hg2+ species. As an example, logb = 7.98 and 15.61 were obtained for (CH3Hg)L-3 - and HgL2- species, respectively (at I approximate to 0.1 mol.L-1 and T = 298.15 K). The dependence of the stability constants on ionic strength, studied through a Debye-Huckel type equation, is also reported. To enrich the thermodynamic study, the enthalpy values of the complex species were obtained by calorimetric titrations at I = 0.1 mol.L-1 in NaCl and T = 298.15 K. Finally, the sequestering ability of HEDPA towards Hg2+ and CH3Hg+ was calculated by means of an empirical parameter, known as pL(0.5), under blood (I = 0.1 mol.L-1, pH = 7.4, T = 310.15 K) and sea water (I = 0.7 mol.L-1, pH = 8.1, T = 298.15 K) conditions. pL(0.5) represents the total ligand concentration required to sequester the 50% of a metal cation present as traces. (C) 2018 Elsevier Ltd.