The phase diagram for the poly(ethylene glycol) dimethyl ether (PEGDE) + Na2SO4 + H2O system at 298.15 K using PEGDE with a molar mass of 2000 was determined. Compositions of the liquid-liquid and the liquid-liquid-solid equilibria were determined using calibration curves of refractive index of the solutions, and atomic absorption (AA) and X-ray diffraction analyses were made on the solids. The solid phase in equilibrium with the biphasic region was anhydrous Na2SO4. An empirical nonlinear three-parameter expression developed by Merchuk was used for reproducing the experimental binodal data at T = (288.15, 298.15, 308.15, and 318.15) K, and the fitting parameters were obtained for the corresponding temperatures. The effects of temperature on the binodal curve were also studied, and it was observed that the area of the biphasic region increased slightly with an increase in temperature. The experimental tie-line compositions at the aforementioned temperatures were fitted to both the Othmer-Tobias and Bancroft and Setschenow-type equations. Correlation coefficients for all equations are reported.