In this work, the thermodynamic properties of the ternary mixed electrolyte system (NaCl + NiCl2 + H2O), using the potentiometric method, are reported. The potentiometric measurements were carried out on the galvanic cell without liquid junction of the type: Na-ISE/NaCl (m(1)), NiCl2 (m(2)), H2O/Ag-AgCl over total ionic strengths from (0.001 to 4.000) mol.kg(-1). A different series of the salt molality ratios r (r = m(NaCl)/m(NiCl2) = 2.5, 5.0,7.5, 10.0) and pure NaCl were used for the measurement at T = 298.2 K. The results obtained were interpreted by using Pitzer ion-interaction model. The Pitzer mixing parameters have been evaluated for the system by including the effects of higher-order electrostatic terms. The Pitzer model is satisfactory and adequate for the description of our aqueous system. Furthermore, the parameters obtained with the Pitzer model were used to calculate the values of the mean activity coefficients of NiCl2, the osmotic coefficients, solvent activity and the excess Gibbs free energy for the whole series of the mixed electrolyte system that was studied. (c) 2011 Elsevier Ltd. All rights reserved.