The phase diagrams of aqueous two-phase systems formed by water, sodium tartrate, and polyethyleneglycol of different molecular weights (600, 1000, 2000, 4000, 6000, and 8000) were determined at 298 K. The effect of NaCl on the liquid-liquid equilibrium was also analyzed. The binodal curves were satisfactorily described using a four-parameter sigmoidal equation. The tie lines were determined by combining a colorimetric method and density measurements. The reliability of the measured tie line compositions was ascertained by the Othmer-Tobias and Bancroft correlation equations.