Abstract: In this study, a new expression for the Helmholtz energy of mixtures containing electrolytes as a function of temperature, density and composition is presented on the basis of perturbation theory. It covers the entire fluid density range from zero to liquid-like densities, and can be used for both phases in vapor-liquid equilibrium calculations. The new expression consists of hard sphere, electrostatic charge and attraction contributions to the Helmholtz energy. For mixtures without electrolytes the equation resembles the equation developed by Hu et al. (1985).For pure molecular components, the parameters are obtained by fitting experimental pure component vapor pressuer, saturated-liquid and saturated vapor density data. The ion-ion interaction parameters are obtained from regressing mean ionic activity coefficient data. The equation of state has been used to correlate mean ionic activity coefficients for twenty-five aqueous electrolyte solutions. Meantime, other four comparable models (Bromley, Meissner, Pitzer, Chen) are compared by using the same set of experimental data. The model presented in this work gives the best representa tions, the Bromley model and the Meissner model are not quite as good, and the Chen model and the Pitzer model give the largest deviation. In addition, the equation of state is applied to predict mean ionic activity coefficients at other temperatures by using the ion-ion interaction parameters at 298.15 K. The results show that the temperature dependence of the parameters is smaller than other four models.