This work presents the effects of changes in temperature (4 degrees C, 25 degrees C, and 40 degrees C), pH (6, 7, and 9), and polymer molecular weight (1000 and 8000) on the equilibrium phase behavior of two-phase PEG + potassium phosphate + water systems. As the temperature and pH were increased, the binodal curve shifted to lower polyethylene and phosphate concentrations, resulting in a higher difference in polymer and salt concentrations between the top and bottom phases. This corresponds to an increase in the region of two-phase coexistence.