In this work, the results of a potentiometric investigation of the thermodynamic properties of the ternary NH4Cl + CaCl2 + H2O mixed electrolyte system are presented. The reported results are based on the experimental data of a galvanic cell, combining a solvent polymeric ammonium ion-selective membrane electrode (NH4+ ISE), and an Ag/AgCl electrode. The potentiometric measurements were performed at 298 K by using different series of mixed salt solutions, each characterized by a fixed salt molal ratio (r = m(NH4Cl)/m(CaCl2) = 0.1, 0.2, 1, and 5), but with similar ionic strengths within the series. The semi-empirical Pitzer ion-interaction theory for mixed salts was used for modeling this ternary electrolyte system over the ionic strength ranging from about 0.05 to 5.5 mol/kg. Besides the determination of the mixed theta(NH4Ca) and psi(NH4CaCl) ion-interaction parameters, the impact of the effect of the higher order electrostatic terms were also analyzed for this mixed ternary electrolyte system. Finally, based on the potentionnetric experimental data and the determined mixing parameters, the calculated values of the osmotic coefficients, the excess Gibbs free energies and the water activities (a(w)) were determined for the whole series of the studied ternary mixed electrolyte system. (c) 2007 Published by Elsevier B.V.