Complexation of uranium(VI) with nitrate was studied at variable temperatures by spectrophotometry and microcalorimetry. The complexation is weak and the stability constant of a 1:1 complex, UO2NO3+, was found to slightly increase as the temperature was increased from 25 to 70 degrees C. The molar enthalpy of complexation at 25 degrees C was determined by microcalorimetry to be small and positive, (3.9 +/- 0.5) kJ . mol(-1), in good agreement with the trend in the stability constant at different temperatures. Discussions were made on topics including weak complexation vs. ionic medium effect and outer sphere vs. inner sphere complexes. Specific ion interaction (SIT) approach was used to obtain the stability constants at infinite dilution and variable temperatures. (C) 2008 Elsevier Ltd. All rights reserved.