A predictive model with three parameters for the calculation of activity coefficients and osmotic coefficients of strong electrolytes in binary solutions is developed. In this development, the Poisson-Boltzmann equation was employed to account for the long-range, ion-ion interactions, and the concept of solvation was employed to account for the short-range, ion-molecule interactions. All three parameters included in the proposed model are expressed clearly with physical explanations. They are (1) the closest distance of approach, a, (2) the equivalent dielectric constant, D(e)BAR and (3) the solvation parameter, S. In the first part of this paper, experimental data obtained from the literature are fitted to the present model. These three parameters are also obtained for 144 aqueous electrolyte solutions and 16 non-aqueous electrolyte solutions. It is found that a is independent of temperature and solvent, while S is temperature independent only. It is shown that the proposed model can be applied conveniently to systems other than those at 298.15 K and to non-aqueous electrolyte solutions. In the second part, this model is extended to predict vapor-liquid equilibria of mixed-solvent electrolyte systems.