Fragments of UO2 fuel pellets extracted from irradiated elements were heated in air at 175 to 275 °C for times up to 800 h. Unirradiated pellets and fragments were studied for comparison. Pretest burnup of the irradiated fuel was typically 190 MW-h/kgU (7900 MWd per tonne of uranium) at a maximum linear power of 45 kW/m. The fuel had been discharged for 1 to 3 yr. The maximum weight gain was at 275 °C, ∼4% in 70 h, indicating 100% conversion to U3O8. The activation energy for the oxidation process at 175 to 275 °C was 130 ± 10 kJ/mol. There was a strong effect of prior irradiation on oxidation rate; the weight gain at 250 °C was about a factor of 6 greater in irradiated compared with unirradiated fuel. There was also an effect of fragment size on oxidation rate. Also, weight gains of fragments from a naturally defected element were less than those for fragments from intact fuel, consistent with prior oxidation in the defected state.