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The phase behavior of potassium sulfate (K2SO4) in polyethylene glycol with molecular weight 8000 (PEG8000) and water (H2O) mixed solvent at 288.15, 298.15, and 308.15 K were determined. According to the results, when the temperature are 288.15 and 298.15 K, there is only a solid-liquid phase equilibrium relationship in the sys-tem, and the phase diagrams are both divided into three parts which respectively are the regions of unsaturated homogeneous liquid (L), one liquid and one solid K2SO4 (L+S) and one liquid and two solids K2SO4 and PEG8000 (L+2S). The solubility of K2SO4 in PEG8000-H2O mixed solvent decreased with the addition of the PEG8000 in the solution. Comparing the diagrams of 288.15 and 298.15 K, the sizes of regions of (L) and (L+S) increased and that of (L+2S) decreased with the increase of temperature. While at 308.15 K, solid-liquid and liquid-liquid equilibrium coexist, and there are six parts in the complete phase diagram at 308.15 K, adding the areas of one liquid and one solid K2SO4 (L+S), two liquids (2 L), two liquids and one solid K2SO4 (2 L+S). The equations developed by Merchuk, Hu, and Jayapal were used to fit the binodal curves data of the system at 308.15 K, mean-while, the experimental tie-line data of the system at 308.15 K were correlated by Othmer-Tobias equation and Bancroft equation.