Liquid-liquid equilibrium (LLE) data were obtained for aqueous two-phase systems composed of polyethylene glycol (PEG) 400 g mol(-1) + organic salt (Na2SO4) + water at different temperatures (293.15, 298.15, 303.15 and 308.15)/K, and pH values of 3.0, 4.0, and 5.0. The effect of temperature and pH on the binodal curves were evaluated, and it was verified that the increase of the variables resulted in a larger biphasic region. The salting out effect was studied using the values of parameters of the effective excluded volume (EEV) model. The experimental results of LLE have been correlated using the UNIFAC model. The root-mean-square deviations between the experimental and predicted LLE compositions for the PEG 400 + Na2SO4 systems were very low. The UNIFAC model was able to represent satisfactorily the aqueous two-phase systems studied.