The Huckel equation used in this study to describe the thermodynamic activity quantities in dilute CaCl2 solution; up to an ionic strength (= I-m) of 1.5 mol.kg(-1) consists of two electrolyte-dependent parameters, B and b(1). Parameter B is linearly related to the ion-size parameter a* in the Debye-Huckel equation, and parameter b(1) is the coefficient of the linear correction term with respect to the molality. In more concentrated solutions up to I-m of 9.0 mol.kg(-1), an extended Huckel equation was used. For it, the Huckel equation was supplemented with a quadratic term in molality, and the coefficient of this term is the parameter b(2). The values of parameters B and b(1) for dilute CaCl2 solutions were determined from the isopiestic data of Robinson for solutions of this salt against KCl solution; (Trans. Faraday Soc. 1940, 36, 735-738). These parameters were successfully tested with the cell potential difference data and isopiestic data available in the literature for dilute CaCl2 solutions. For more concentrated solutions, new values of parameters b(1), and b(2) were determined for the extended Huckel equation from the data measured by Spedding et al. (J. Chem Eng. Data 1976, 21, 341-360) for isotonic KCl and CaCl2 solutions, but the same value of the parameter B was used as that for dilute solutions. The resulting parameter values were tested with the existing vapor pressure and isopiestic data, and these data support well the new values. Reliable thermodynamic activity quantities for CaCl2 solutions are, therefore, obtained by using the new Huckel and extended Huckel equations. The activity coefficients, osmotic coefficients, and vapor pressures resulting from these equations are tabulated here at rounded molalities. These values were compared to those reported in the several previous tabulations.