Abstract: In this paper, 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. The new expression consists of hard sphere, electrostatic force and attraction contributions to the Helmholtz energy. For pure molecular components, the parameters are obtained by fitting experimental pure component vapor pressure, saturated-liquid density and PVT data. The ion-ion interaction parameters are determined from regression of mean ionic activity coefficient data.The solubility data for gas-water-salt systems are correlated by using only one adjustable parameter. Average absolute deviations of calculated bubble point pressures are less than 15% for most systems studied. The results are satisfactory.