The hydrolysis of uranium (VI) in tetraethylammonium perchlorate (0.10 mol dm(-3) at 25 C) was studied at variable temperatures (10-85 degreesC). The hydrolysis constants (*betan,m) and enthalpy of hydrolysis (DeltaHn,m) for the reaction MUO22+ + nH(2)O = (UO2)(m)(OH)n((2M-n)+) + nH(+) were determined by titration potentiometry and calorimetry. The hydrolysis constants, *beta(1,1),*beta(2,2),and *beta(5,3), increased by 2-5 orders of magnitude as the temperature was increased from 10 to 85 degreesC. The enthalpies of hydrolysis, DeltaH(2),(2) and AH(5,3), also varied: DeltaH(2,2) became more endothermic while DeltaH(5,3) became less endothermic as the temperature was increased. The heat capacities of hydrolysis, DeltaC(p(2,2)) and DeltaC(p(5,3)), were calculated to be (152 +/- 43) J K-1 mol(-1) and -(229 +/- 34) J K-1 mol(-1), respectively. UV/Vis absorption spectra supported the trend that hydrolysis of U(VI) was enhanced at elevated temperatures. Time-resolved laser-induced fluorescence spectroscopy provided additional information on the hydrolyzed species at different temperatures. Approximation approaches to predict the effect of temperature were tested with the data from this study.