This work reports the results of a thermodynamic investigation of the ternary NaCl-MgCl2-H2O mixed electrolyte system. The potentiometric measurements were performed at 298 K, using a galvanic cell by combining a solvent polymeric sodium ion-selective membrane electrode (Na+ ISE), containing a mixture of NAP-dibenzyl-NAP-diphenyl-1,2-phenylene-dioxydiacetamide as ionophore, and an Ag/AgCl electrode. Different series of mixed salt solutions, each characterized by a fixed salt molal ratio r (r = M-NaCl/m(MgCl2) = 0.100, 0. 199, 0.995, 4.990 and 10.364) but with similar ionic strengths, were used in this investigation. The modeling of the ternary electrolyte system was based on the Pitzer ion-interaction semi-empirical theory for mixed salts, over the ionic strength ranging from 0.05 to 4.3 mol/kg. Pitzer ion-interaction parameters for mixed salts including the effect of the higher-order electrostatic terms were determined for this system. Moreover, based on the potentiometric experimental data and the determined mixing parameters, the values of the mean activity coefficients, the osmotic coefficients, the excess Gibbs free energies and the water activities (a(w)) were reported for the whole series of the studied ternary mixed electrolyte systems. Finally, the impact of the magnitude of the potentiometric selectivity coefficient of this Na+ ISE on the quality of the potentiometric data revealed that Nernst equation could suitably be used for the evaluation of mean activity coefficients (with at least 3 decimal numbers), for the ionic strength beyond 0.25 mol/kg and for mixed electrolyte systems with molal salt ratio higher than r = m(NaCl)/m(MgCl2) >= 5. (c) 2007 Elsevier B.V All rights reserved.