This paper presents a predictive model for phase equilibria calculations in the presence of electrolyte solutions. The model is based on a thermodynamic approach in which an equation of state is combined with a modified Debye-Huckel electrostatic term, with only one adjustable parameter for the water-rich phase. The water-salt parameter has been determined by using vapour pressure depression data at 373.15 K and freezing point depression data of single salt solutions. NaCl, KCl, CaCl2, Na2SO4, NaF, NaBr, MgCl2, SrCl2, BaCl2 and their mixtures, have been modelled. Although the work is primarily aimed at gas hydrate inhibition effect of formation water in subsea gathering networks and transmission lines, other properties can also be predicted over a wide range of temperatures (250-423 K) and salt concentrations (close to saturation). These data include vapour pressure and freezing point depressions, hydrate inhibition of pure, and multi-component systems in the presence of single and mixed electrolyte solutions. The predictions of the model have been compared with experimental data and good agreement has been demonstrated.