A combination of simultaneous thermal analysis, evolved gas analysis and non-ambient XRD techniques was used to characterise and investigate the conversion reactions of ammonium uranates into uranium oxides. Two solid phases of the ternary system NH3 - UO3 - H2O were synthesised under specified conditions. Microspheres prepared by the sol-gel method via internal gelation were identified as 3UO3 - 2NH3 - 4H2O , whereas the product of a typical ammonium diuranate precipitation reaction was associated to the composition 3UO3 - NH3 - 5H2O. The thermal decomposition profile of both compounds in air feature distinct reaction steps towards the conversion to U3O8, owing to the successive release of water and ammonia molecules. Both compounds are converted into α-U3O8 above 550 °C, but the crystallographic transition occurs differently. In compound 3UO3 - NH3 - 5H2O (ADU) the transformation occurs via the crystalline β-UO3 phase, whereas in compound 3UO3 - 2NH3 - 4H2O (microspheres) an amorphous UO3 intermediate was observed. The new insights obtained on these uranate systems improve the information base for designing and synthesising minor actinide-containing target materials in future applications.