Liquid-liquid equilibrium data of aqueous two-phase systems (ATPS) composed of polyethylene glycol (PEG) 8000 + sodium sulfate + water, PEG 8000 + ammonium sulfate + water, and PEG 8000 + magnesium sulfate + water at T = 298.15, 308.15, and 318.15 K and pH 2.0 were determined. The universal function activity coefficient (UNIFAC) model was correlated to the experimental tie-line data, and the root-mean-square deviations (RMSDs) between experimental and calculated data were considered in the calculation. The effects of temperature and cation type on the spinodal curves were evaluated. The salting-out effect was studied by the determination of the effective excluded volume (EEV). Rising temperature resulted in an increase in the biphasic region and tie-lines. Regarding the cation type, the phase-separation abilities followed the order Mg2+ > Na+ > (NH4)(+). Increases in the overall composition of the systems resulted in increased tie-line lengths. The tie-line slope absolute values also tended to rise with increasing temperature. Increasing the ATPS constituent concentrations resulted in an increase in the phase densities. The increase in temperature resulted in an increase in the salting-out effect. The exclusion volume values increased in the following order: (NH4)(+) < Na+ < Mg2+. The overall mean deviation between the experimental and correlated LLE compositions for the PEG 8000 + ammonium sulfate + water, PEG 8000 + magnesium sulfate + water, and PEG 8000 g mol(-1) + sodium sulfate + water was 3.901%. The results of the UNIFAC model agree with the experimental tie-line values. This allows the use of this model to obtain reliable data for this system, thus reducing the number of experiments to be performed for a processing design.