Pitzer’s ion interaction parameters for aqueous barium chloride are derived as a function of temperature from published parameter values for the activity coefficient and for the enthalpy at 298.15 K, and by fitting osmotic coefficients measured by Holmes and Mesmer (J. Chen. Thermodyn. 1981, 13, 1025) to 473.61 K. This allows the calculation of the thermodynamic properties of barium chloride solutions including the BaCl2(aq) activity coefficient and the activity of water. Barium chloride mono- and dihydrate solubilities in pure water are used to calculate their solubility products at various temperatures. Solubility, data of barium chloride dihydrate in the Na-K-Ca-Ba-Cl-H2O system are used to calculate Fitter’s mixing parameters. It is shown that the solubility of BaCl2 . 2H(2)O(s) in this system can be accurately predicted to high concentration using the resulting parameters.