Complexation of U(VI) with fluoride at elevated temperatures in aqueous solutions was studied by spectrophotometry, Four successive complexes, UO2F+, UO2F2(aq), UO2F3-, and UO2F42-, were identified, and the stability constants at 25, 40, 55, and 70 degrees C were calculated. The stability of the complexes increased as the temperature was elevated. The enthalpies of complexation at 25 degrees C were determined by microcalorimetry. Thermodynamic parameters indicate that the complexation of U(VI) with fluoride in aqueous solutions at 25 to 70 degrees C is slightly endothermic and entropy-driven. The Specific Ion Interaction (SIT) approach was used to obtain the thermodynamic parameters of complexation at infinite dilution. Structural information on the U(VI)/fluoride complexes was obtained by extended X-ray absorption fine structure spectroscopy.