Carboreduction of UO3 has been studied by in situ infrared spectroscopy in order to detect the presence of carbon monoxide during the reduction. From thermogravimetry, the reaction was observed to occur in two steps: (i) reduction of UO3 to U3O8 by carbon at 440°C and (ii) reduction at 590°C of U3O8 to UO2 by carbon. No carbon monoxide was observed during the reaction. Nevertheless, formation of carbonate species is suggested at temperature above 550°C with absorption bands around 1005 and 1300 cm−1.