The Huckel equation used in this study for the thermodynamic activity quantities in dilute MgCl2 solutions up to an ionic strength (=I-m) of 1.5 mol . kg(-1) contains two parameters being dependent on the electrolyte, i.e., those of B and b(1). The former is linearly related to the ion-size parameter in the Debye-Huckel equation and the latter is the coefficient of the linear correction term with respect to the molality. For 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 extended with a quadratic term in molality, and the coefficient of this term is the third parameter b(2). Parameters B and b(1) for dilute MgCl2 solutions were obtained from the isopiestic data of Robinson and Stokes for solutions of this salt and KCl [Trans. Faraday Soc. 36 (1940) 733] by using the previous Huckel parameters for dilute KCl solutions [J. Chem. Eng. Data 54 (2009) 208]. The resulting parameters for MgCl2 solutions were successfully tested with all isopiestic data available in the literature for dilute solutions of this salt. For less dilute solutions, new values for parameters b(1) and b(2) were determined for the extended version of the Huckel equation of MgCl2 solutions from the isopiestic data of Rard and Miller [J. Chem. Eng. Data 26 (1981) 38] for NaCl and MgCl2 solutions but the dilute-solution value for parameter B was used. The previous extended Huckel equation for concentrated NaCl solutions was used in this estimation (see the KCl citation above). In the tests of the new parameter values for these less dilute solutions, the existing vapor pressure and isopiestic data were used, and these data support well the new parameters. Reliable thermodynamic activity quantities for MgCl2 solutions can, therefore, be obtained using the new Huckel and extended Huckel equations. The activity coefficients, osmotic coefficients, and vapor pressures obtained with these equations are tabulated here at rounded molalities. These values were compared to those reported in the several previous important tabulations. (c) 2013 Elsevier Ltd. All rights reserved.