A modified Fitter’s virial equation, which combines the long-range electrostatic contribution from Fowler and Guggenheim with the short-range virial equation, is presented for the excess Gibbs free energy of some special kinds of systems. The model is used to predict the liquid-liquid equilibrium (LLE) at 298.15 K of three polymer-uni-bivalent salt-aqueous two-phase systems : polyethylene glycol(PEG)-(NH4)(2)SO4, PEG-Na2SO4, PEG-Na2CO3 by correlating the parameters from the corresponding vapor-liquid equilibrium (VLE) data. The results show that the model can give excellent representation of the VLE data of PEG solutions, either binary or ternary, the model parameters being independent of the chain lengths of the polymers. The comparison between the predicted and experimental phase diagrams shows that the suggested model has good prediction ability for PEG-uni-bivalent salt systems.