Liquid-liquid equilibrium data of aqueous two-phase systems composed of polyethylene(glycol) (PEG) 4000 + zinc sulfate at (5, 10, 35, and 45) degrees C were measured. The effect of temperature in the systems was studied. The expansion of the two-phase area with the increase in temperature from (5 to 35) degrees C and also a small reduction of the biphasic region from (35 to 45) degrees C were observed. The slope of all tie lines increased with the temperature increase. The tie line length increased in the ranges of (5 to 25) degrees C and (35 to 45) degrees C and decreased from (25 to 35) degrees C.