The aqueous two-phase system (ATPS), a polymer and a salt, is widely used as an extraction process for separation of biomolecules in down stream processing. At this work, using local area fraction, the local composition non-random factor (NRF) model of Haghtalab and Vera is extended for aqueous binaries of electrolyte, polymer and ternaries of aqueous polymer-salt solutions. Using area fraction of species, a new function of excess Gibbs energy has been developed for multicomponent aqueous polymer-electrolyte systems. The modified NRTL-NRF model is applied to correlate the phase behavior of aqueous two-phase polymer-salt systems of PEG + Na2SO4 + H2O, PEG + K2HPO4 + H2O and PEG + (NH4)(2)SO4 + H2O at 25 degrees C. The effect of pH solution and molecular weight of polyethylene glycol (PEG) is investigated and the tie lines with binodal curves for the fifteen systems are shown. The comparison of the results of the present model with the experiment shows that the modified NRTL-NRF model is more accurate. (C) 2008 Elsevier B.V. All rights reserved.