The phase diagram was determined for the PEG + Cs2SO4 + H2O system at 25 degreesC using PEG (poly(ethylene glycol)) with a molecular weight of 4000. Compositions of the liquid-liquid and the liquid-liquid-solid equilibrium were determined using calibration curves for the index of refraction of the solutions; atomic absorption (AA) and TG analyses were made on the solids. The solid phase in equilibrium with the biphasic region was Cs2SO4. The binodal curves were correlated using a four-parameter equation. Tie lines were satisfactorily described using the Othmer-Tobias and Bancroft equations. The effects of temperature (25 degreesC, 35 degreesC and 45 degreesC) and molecular weight of the PEG (1000, 4000, and 10 000) on the binadol curve were also investigated. It was observed that the effect of the temperature was insignificant within the investigated range and an increase in the molecular weight of the PEG produced a displacement of the binodal curve toward the origin.