The surface oxidation of UO2 sintered plates at 170-275 ° C was studied in situ by high temperature X-ray diffractometry. At very low oxygen concentration, UO2 is oxidized to U4O9, while at 300°C and argon-20 vol% oxygen it is oxidized up to U3O7. X-ray diffraction profiles of the UO2, U4O9 and U3O7 phases were well characterized during the transformations. The activation energy for the transformation of UO2 to U4O9, obtained from X-ray diffraction data, was found to be 117 ± 9 kJ/mol and 90 ± 14 kJ/mol for the β-(311) and α-(200) reflections, respectively. © 1991.