The equilibrium behaviors of two-phase liquid liquid systems composed of poly(ethylene glycol) (PEG) 1500 or 4000 + sodium sulfite + water were experimentally determined at temperatures of (288.15, 298.15, 308.15, and 318.15) K. The effects of the molecular weight of PEG and the temperature on the phase separation were studied. The binodal curves were fitted to an empirical equation that correlates the concentrations of PEG 1500 or 4000 and sodium sulfite, and the coefficients for the different temperatures were estimated. The tie-line compositions were estimated and correlated using the Othmer-Tobias and Bancroft equations, and the parameters are reported. The liquid liquid equilibrium (LLE) experimental data obtained were well-correlated to the activity coefficients of the non-random two-liquid (NRTL) and UNIversal QUAsiChemical (UNlQUAC) models, and the mean deviations were less than 0.36 % and 0.31%, respectively.