The reduction of uranium oxide U3O8 into uranium dioxide UO2 has been studied by temperature-programmed thermogravimetry, up to 700°C. Experiments have been carried out either in ammonia (PNH3 ranging from 35 to 125 hPa) or hydrogen (PH2 ranging from 42 to 243 hPa). The gases evolved and consumed during the reduction were followed simultaneously by mass spectrometry. The reduction of U3O8 by ammonia into UO2 begins at 550°C, and is completed at about 650°C. It has been noticed that ammonia decomposition occurs at 700°C; moreover, it is catalysed by UO2 produced by the reduction of U3O8, since no decomposition is observed in the absence of UO2. Besides, some isothermal experiments carried out at 510°C have confirmed that ammonia reacts directly with U3O8 since the shape of the curves obtained either in ammonia or in hydrogen are different, particularly, the reaction is faster with ammonia compared to hydrogen, for the same partial pressure of the reducing gas.