Water activities in the ternary system (CaCl2 + SrCl2 + H2O) and its sub-binary system (CaCl2 + H2O) at T = 298.15 K have been elaborately measured by an isopiestic method. The data of the measured water activity were used to justify the reliability of solubility isotherms reported in the literature by correlating them with a thermodynamic Pitzer-Simonson-Clegg (PSC) model. The model parameters for representing the thermodynamic properties of the (CaCl2 + H2O) system from (0 to 11) mol . kg (1) at T = 298.15 K were determined, and the experimental water activity data in the ternary system were compared with those predicted by the parameters determined in the binary systems. Their agreement indicates that the PSC model parameters can reliably represent the properties of the ternary system. Under the assumption that the equilibrium solid phases are the pure solid phases (SrCl2 center dot 6H(2)O and CaCl2 center dot 6H(2)O)(s) or the ideal solid solution consisting of CaCl2 center dot 6H(2)O(s) and SrCl2 center dot 6H(2)O(s), the solubility isotherms were predicted and compared with experimental data from the literature. It was found that the predicted solubility isotherm agrees with experimental data over the entire concentration range at T = 298.15 K under the second assumption described above; however, it does not under the first assumption. The modeling results reveal that the solid phase in equilibrium with the aqueous solution in the ternary system is an ideal solid solution consisting of SrCl2 center dot 6H(2)O(s) and CaCl2 center dot 6H(2)O(s). Based on the theoretical calculation, the possibility of the co-saturated points between SrCl2 center dot 6H(2)O(s) and the solid solution (CaCl2 center dot 6H(2)O + SrCl2 center dot 6H(2)O)(s) and between CaCl2 center dot 6H(2)O(s) and the solid solution (CaCl2 center dot 6H(2)O + SrCl2 center dot 6H(2)O)(s), which were reported by experimental researchers, has been discussed, and the Lippann diagram of this system has been presented. (C) 2013 Elsevier Ltd. All rights reserved.