A quantitative analytical method for the study of the liquid-liquid equilibrium (LLE) of K2HPO4-KH2PO4-PEG aqueous two-phase quaternary systems has been established. LLE data for four series of experiments at different molecular weights PEG (1000, 2000, 4000 and 6000) with pH = 9.43 and three series of experiments at different pH values (6, 7 and 8) with PEG4000 at 25-degrees-C were measured. The association constants of K2HPO4, (NH4)2SO4 and KH2PO4 in water were determined also. The extended UNIFAC equation combined with the Fowler-Guggenheim equation are proposed for the correlation and prediction of the LLE data. The results are satisfactory.