Liquid-liquid equilibria of an aqueous two-phase system consisting of poly(ethylene oxide) 1500, sodium nitrate, and water at 293.15, 313.15, and 333.15 K were studied. Binodal data and a complete phase diagram with equilibrium liquid-phase compositions were obtained by using conductometry and refractometry techniques. The equilibrium data for aqueous two-phase systems were correlated using the UNIQUAC model, and new binary interaction parameters were calculated.