Liquid−liquid and solid−liquid equilibrium data are presented for the PEG 4000 + Na2SO4 + water system at 298.15 K. Six equilibrium regions were found. The binodal curve was fitted by means of a three-parameter equation (relative mean deviation 3.95%), and the tie lines compositions were fitted to both the Othmer−Tobias and Bancroft equations. Densities and refractive indices were determined for the solutions in equilibrium. Samples of the solid phase analyzed by X-ray diffraction showed it to consist of anhydrous Na2SO4.