In the present study, an activity coefficient model, based on the concept of local volume fractions and the Gibbs-Helmholtz relation, has been developed. Some modifications were made from Tan-Wilson model (1987) and TK-Wilson model (1975) to represent activity coefficients in mixed solvent-electrolyte systems. The proposed model contains two groups of binary interaction parameters. One group for solvent-solvent interaction parameters corresponds to that given by the TK-Wilson model (1975) in salt-free systems. The other group of salt-solvent interaction parameters can be calculated either from vapor pressure or bubble temperature data in binary salt-solvent systems. It is shown that che present model can also be used to describe liquid-liquid equilibria. No ternary parameter is required to predict the salt effects on the vapor-liquid equilibria (VLE) of mixed solvent systems, By examining 643 sets of VLE data, the calculated results show that the prediction by the present model is as good as that by the Tan-Wilson model (1987), with an overall mean deviation of vapor phase composition of 1.76% and that of the bubble temperature of 0.74 K.