The thermal decomposition of ammonium diuranate (ADU) in air is investigated using in-situ high-temperature x-ray diffraction (HT-XRD), thermogravimetry, and differential thermal analysis (TG/DTA). Data have been collected in the temperature range from 30 to 1000 °C, allowing to observe the sequence of phase transformations and to assess the energy changes involved in the calcination of ADU. The starting material 2UO3•NH3•3H2O undergoes a process involving several endothermic and exothermic reactions. In situ HT-XRD shows that amorphous UO3 is obtained after achieving complete dehydration at 300 °C, and denitration at about 450 °C. After cooling from heat treatment at 600 °C, a crystalline UO3 phase appears, as displayed by ex-situ XRD. The self-reduction of UO3 into orthorhombic U3O8 takes place at about 600 °C, but a long heat treatment or higher temperature is required to stabilize the structure of U3O8 at room temperature. U3O8 remains stable in air up to 850 °C. Above this temperature, oxygen losses lead to the formation of U3O8-x, as demonstrated by subtle changes in the diffraction pattern and by a mass loss recorded by TGA. ; JRC.E.6-Actinide research