The new experimental data of liquid-liquid equilibria for aqueous two-phase systems PEG-K2HPO4-water and PEG-Na2SO4-water are presented. The effects of pH and molecular weight of polyethylene glycol were investigated and the tie lines with binodal curves for both systems are shown. A new thermodynamic model based on group contribution has been proposed for studying the phase behavior of aqueous two-phase polymer-salt systems. The assumptions of NRTL-NRF model and the activity coefficient equation of UNIQUAC-NRF model have been used for the groups. In this new model, UNIFAC-NRF, the nonrandom state of groups were selected as a reference state. The binary interaction parameters were adjusted using the data of binary salt-water systems and the ternary systems were correlated with only six binary adjustable parameters. The Debye-Huckel equation based on Fowller-Guggenheim equation was used to calculate the long range electrostatic interaction of the ions. The UNIFAC-NRF model was applied to correlate the experimental data of aqueous two-phase systems: PEG-K2HPO4-water and PEG-Na2SO4-water for two different molecular weight of PEG at different pH. The results of the new model showed that it can be used to correlate the LLE in aqueous solution of polymer-salt very well. (C) 2003 Elsevier B.V. All rights reserved.