Phase diagrams and liquid-liquid equilibrium (LLE) data of the aqueous polyethylene glycol (PEG)-sodium dihydrogen phosphate and aqueous PEG-disodium hydrogen phosphate systems have been determined experimentally at 298.15, 308.15 and 318.15 K for two different molecular weights of the polymer (1000 and 6000). Furthermore, the osmotic virial equation was used to correlate the phase behavior of these systems. Good agreement was obtained with the experimental data. Finally, the effect of temperature, the polymer molecular weight and the type of salt on the LLE are discussed.