Vapor-liquid equilibrium (VLE) data are reported for binary and mixed electrolyte solutions containing CaCl2, MgCl2, K2SO4, MgSO4, and KCl at total molality of salt between (0.0 and 12.0) mol.kg(1). The measurements were carried out with a pressure between (6.3 and 101.3) kPa in a computer-controlled glass apparatus. An activity coefficient model based on the hypothesis of hydration was deduced in this paper. Experimental data and literature data for 20 salts, with temperature spanning from (273.15 to 415.85) K, were successfully correlated using the developed model. Meanwhile, the model was also successfully applied to predict the VLE data in mixed electrolyte solution systems with the binary parameters.