Liquid−liquid equilibrium data and phase diagrams of aqueous two phase systems (ATPS) composed of polyethylene glycol (PEG) of molar mass (1500 and 4000) g•mol −1 + sodium sulfate + water were determined at temperatures of 293.15, 303.15, 313.15, and 323.15 K at pH 2. Temperature had no significant effect on the biphasic region, indicating a small enthalpic contribution in the phase separation process. The change in temperature however caused an increase in the tie line slope because water molecules were transferred from the upper phase to the lower phase. Increasing the molar mass of the polymer led to an increase in the biphasic region of the phase diagram. In determining the physical properties of the systems studied, it was observed that the increase in concentration increased the viscosity and the refraction index; while relative to temperature, the viscosity, refraction index, and density decreased.