Abstract: A new equation of state(PHSMSA EOS)based on perturbation theory is developed for calculating high-pressure phase equilibria of aqueous electrolyte systems containing supercritical gases,light hydrocarbons andpolar components.The binary interaction parameters are determined:for ion-ion pairs by regression of ionicactivity coefficient data;for molecule-molecule pairs by fitting the VLE data of binary nonelectrolyte mixtures;and for ion-molecule pairs by fitting the gas solubility data of ternary gas-water-salt systems.The new EOShas been tested on the prediction of solubilities of methane,nitrogen and natural gas mixtures in brine.Sat-isfactory agreement with the experimental data measured by authors and other investigators is observed.