The water activities in aqueous mixtures containing ethylene oxide-propylene oxide random copolymer (EOPO) with different molecular weights, both binary and ternary systems, were measured using the improved isopiestic method. The modified NRTL model was extended to predict the phase behavior in polymer + salt aqueous two-phase systems at 298.15 K. The model parameters were evaluated directly from the experimental water activities. The phase diagrams of LLE of EOPO + (NH4)(2)SO4 aqueous two-phase systems with different molecular weights were successfully predicted using only one group of model parameters. Good agreement with experimental data is obtained, with most of average absolute deviations being less than 2.2 w/w %.