Sulfate, one of the inorganic constituents in the groundwater of nuclear waste repository, could affect the migration of radioactive materials by forming complexes. Spectrophotometric and microcalorimetric titrations were performed to identify the U(VI)/sulfate complexes and determine the equilibrium constants and enthalpy of complexation at (25 to 70) degrees C. Results show that U(VI) forms moderately strong complexes with sulfate, i.e., UO2SO4(aq) and UO2(SO4)(2)(2-), in this temperature range and the complexes become stronger as the temperature is increased: 2-fold and 10-fold increases in the stability constants of UO2SO4(aq) and UO2{SO4}(2)(2-), respectively, when the temperature is increased from (25 to 70) degrees C. The complexation is endothermic and entropy-driven, showing typical characteristics of inner-sphere complexation and "hard acid"/"hard base" interactions. The thermodynamic trends are discussed in terms of dehydration of both the cation (UO22+) and the anion (SO42-) as well as the effect of temperature on the structure of water. (C) 2008 Elsevier Ltd. All rights reserved.