The phase behaviors of aqueous UO2SO4 solutions were investigated in situ with a microscope and a Raman spectrometer at temperatures from 25 to 420 degrees C. Results show that aqueous UO2SO4 solution separated into UO2SO4-rich (U-rich) and UO2SO4-poor (U-poor) liquid phases coexisted with a vapor poor, phase at >= 285.8 +/- 0.5 degrees C. Both visual and Raman spectroscopic investigations suggest that a reversible strong UO22+-SO42- association was responsible for the liquid-liquid immiscibility in aqueous UO2SO4 solutions. Main evidences were summarized as: (1) the liquid-liquid phase separation temperature decreases with increasing UO2SO4 concentration up to 0.54 mol/kg, and then increased at greater concentrations, characterizing a lower critical solution temperature (LCST) at 285.8 degrees C +/- 0.5 degrees C. LCST is commonly accepted as a diagnostic feature of polymer solutions; (2) analyses of the shapes of the Raman spectra of nu(1) (UO22+) and nu(1) (SO42-) bands show that the UO22+-SO42- association becomes stronger at elevated temperatures, especially in the immiscible U-rich, phase; and (3) with increasing temperature, the U-rich phase becomes more concentrated, whereas the U-poor phase becomes more dilute, indicating that the hydration of UO22+ and SO42- cannot be maintained in the U-rich phase. Destruction of the hydration spheres of UO22+ and SO42- further favors the ion association in the U-rich, phase. These results are important for describing similar sulfate solutions at elevated temperatures, especially under supercritical conditions. (C) 2016 Elsevier B.V. All rights reserved.