As a part of the DUPIC fuel development program, oxidation behavior of the simulated spent fuel with dissolved fission products has been studied using a thermogravimetry analyzer in the temperature range of 573-873 K in air in order to establish the oxidation temperature and time of the OREOX process during fabrication of DUPIC fuel. Oxidation behavior of an UO2 sintered pellet has also been measured for the purpose of comparison. From the XRD study, the simulated spent fuel has been converted to U3O8, in the temperature range of 573-873 K. The oxidation curves of the simulated spent fuel are displayed sigmoidal reaction kinetics which follows the nucleation-and growth mechanism of the formation of U3O8. The fission products forming the solid solution in UO2 matrix delayed U3O8 forming. Activation energies of the simulated spent fuel are higher than those of UO2. There are transition points in the rate of the oxidation of the simulated fuel and UO2 between 673 and 723 K, and the activation energies in the low temperature range of 573-673 K are higher than those in the high temperature range of 723-873 K. (C) 2006 Elsevier B.V. All rights reserved.