Liquid-liquid equilibrium data and phase diagrams of new aqueous two-phase systems (ATPS's), composed of water + sodium hydroxide + poly(ethylene glycol) (PEG) of molar masses (1450, 4000, or 10 000) g.mol(-1), were determined at (288.2, 298.2, and 308.2) K. The effects of temperature and PEG molar mass on the equilibrium data were studied. It was found that an increase in temperature caused the expansion of the two-phase region and increased the slope and the length of the tie line. It was also observed that binodal curves shifted toward the origin with the increase in PEG molar mass.