Water activity for binary KCl-H2O, NaCl-H2O and MgCl2-H2O, as well as two ternary NaCl-MgCl2-H2O and KCl-MgCl2-H2O, systems has been measured by using an isopiestic method at 323.15 K. The isopiestic results obtained show that the isopiestic composition lines of the NaCl-MgCl2-H2O system was found to obey the Zdanovskii rule, whereas the KCl-MgCl2-H2O system was observed to deviate slightly. The experimental water activities determined were applied to regress the parameters of the Pitzer model with a good agreement. The model with new parameters is validated by comparing water activity predictions with those given in the literature and not used in the parametrization process and calculating the solubility of the NaCl-MgCl2-H2O and KCl-MgCl2-H2O systems at various temperatures with the comparison of literature values.