Phase diagrams and liquid-liquid equilibrium (LLE) data of the poly(ethylene glycol) (PEG) (M-n: 6000) + sodium hydroxide system have been determined at (298.15, 308.15, and 318.15) K. As temperature is increased, this aqueous two-phase system exhibits a phase inversion. Specifically, the PEG-rich phase-which at low temperatures is the less dense top phase-becomes the lower phase at elevated temperatures. The effect depends on the concentration of both PEG and sodium hydroxide. At the temperatures 308.15 K and 318.15 K, the system shows inversion; and at these temperatures inversion occurs at lower concentrations of either phase-forming constituent. Also, it was found that as temperature is increased the inversion-phase region expands. The osmotic virial equation with temperature-dependent parameters was used to correlate the phase behavior of this system.