A combination of simultaneous thermal analysis, evolved gas analysis and non-ambient XRD techniques was used to characterise and investigate the thermal decomposition behaviour in the NH3 − UO3 − H2O class of materials. One compound was prepared according to a typical ammonium diuranate precipitation reaction, and could be identified as 3UO3·NH3·5H2O. Microspheres prepared by the sol-gel method via internal gelation were associated to the composition 3UO3·2NH3·4H2O under the specified conditions. The products were analysed using the techniques listed below, the resulting data are part of this dataset. - TGA, combined with EGA-MS (Tmax = 1300 °C, heating rate = 2 °C/min) - TG-DSC, combined with EGA-MS (Tmax = 1300 °C, heating rate = 10 °C/min) - ambient XRD (dried products after synthesis) - in-situ high temperature XRD (including initial and final scans, taken at 35 °C) - Tmax for 3UO3·NH3·5H2O = 1300 °C; Tmax for 3UO3·2NH3·4H2O = 650 °C - Samples measured directly on a Pt/Rh heating strip (Pt/Rh phase visible in patterns, blank scan included)