Phase diagrams and models of PEG 6000, PEG 8000, and PEG 10000 in water and potassium sodium tartrate systems were constructed at temperatures 10, 25, and 45 degrees C. These phase diagrams consist of binodal curves and tie-lines, modeled using the Merchuck equation, and validated using a linearized form of the Othmer-Tobias equation. It was observed that higher system temperatures produced larger binodal envelopes and better phase separation. It was also observed that smaller PEG molecular weights produced smaller binodal envelopes.