An equation-of-state (EOS) module has been developed for the TOUGH2 simulator, belonging to the MULKOM family of computer codes developed at Lawrence Berkeley National Laboratory. This module, named EWASG (Equation-of-State for Water, Salt and Gas), is able to handle three-component mixtures of water, sodium chloride, and a slightly soluble noncondensible gas (NCG). At present the NCG can be chosen to be air, CO2, CH4, H-2, or N-2. EWASG can describe liquid and gas phases, and includes precipitation and dissolution of solid salt. The dependence of density, viscosity, enthalpy, and vapour pressure of brine on salt concentration is taken into account, as well as the effects of salinity on gas solubility in the liquid phase and related heat of solution. The reduction of rock porosity because of salt precipitation is also considered, as well as the related decrease of formation permeability. Vapour pressure lowering (VPL) due to suction pressure is represented by Kelvin＇s equation, in which the effects of salt are considered whereas those of NCG have currently been neglected. The main assumptions made in developing the EWASG module are described, together with the correlations employed to calculate the thermophysical properties of multiphase H2O-NaCl-CO2 mixtures, which can be used to simulate the thermodynamic behaviour of commonly exploited geothermal reservoirs.