This work reports the results of a thermodynamic investigation of the ternary mixed-electrolyte system (CsCl + CaCl2 + H2O). The activity coefficients of this mixed aqueous electrolyte system have been studied with the electromotive force measurement (EMF) of the cell: Cs ion-selective electrode (ISE)vertical bar CsCl(m(A)), CaCl2(m(B)), H2O vertical bar Ag/AgCl at T = 298.15 K and over total ionic strengths from (0.01 to 1.50) mol . kg(-1) for different ionic strength fractions y(B) of CaCl2 with y(B) = (0, 0.2, 0.4, 0.6, and 0.8). The cesium ion-selective electrode (Cs-ISE) and the Ag/AgCl electrode used in this work were made in our laboratory and had a good Nernst response. The experimental results obey the Harried rule, and the Pitzer model can be used to describe this ternary system satisfactorily. The osmotic coefficients, excess Gibbs free energies and activities of water of the mixtures were also calculated. (C) 2009 Elsevier Ltd. All rights reserved.