Liquid–liquid equilibrium data of aqueous two phase systems (ATPS) composed of polyethylene glycol (PEG) 1500 g·mol–1 + sodium sulfate + water at T = (293.15, 303.15 and 313.15) K and p = 0.1 MPa were determined. The density of the top and bottom phases of tie-lines (TL) was determined. The universal functional activity coefficient (UNIFAC) model was correlated to the experimental tie-line data, and the root-mean-square deviations (RMSD) between experimental and calculated data were considered in the calculation. The salting-out effect was evaluated using the model of the effective volume (EEV). The temperature had no influence on the binodal curves and TL, this effect can be attributed to a small enthalpic contribution in the phase separation process. Increases in the ATPS constituents concentrations resulting in an increase in the density of the phases. The increase in temperature resulted in an increase in the salting-out effect. The highest estimated value of EEV was obtained for a system at 293.15 K. Calculated RMSD between the experimental and predicted LLE compositions for the PEG 1500 + sodium sulfate systems was 1.71%. The results of the UNIFAC model agree with the experimental tie-line values. The use of this thermodynamic model allowed the acquisition of reliable data for the system, being able to reduce the number of experiments performed.