The (solid + liquid) equilibrium (SLE) and (liquid + liquid) equilibrium (LLE) of RbCl/CsCl + ethylene carbonate (EC) + H2O ternary systems were carefully investigated at 298.15 K. The SLE and LLE data were correlated by the corresponding equations respectively. Furthermore, the thermodynamic properties of RbCl/CsCl in the EC-water systems were determined by potentiometric method at T = 298.15 K. The experiment data were correlated by the Pitzer, modified Pitzer and the extended Debye-Huckel equations. The mean activity coefficients, osmotic coefficients and excess Gibbs free energies, standard Gibbs free energy of transfer and the standard solubility product of systems were calculated. The mean activity coefficient gamma(+/-) decreases with the increase of the molality of electrolyte at the fixed mass fraction of EC. But gamma(+/-) vary abnormally with the increase of the EC content. The result of Delta G(t)(0) reflects the spontaneously transfer of the electrolytes from water to the mixed solvents. (C) 2016 Elsevier Ltd. All rights reserved.