The thermal decomposition process of Ce(IV)(NH4)(2)(NO3)(6) has been determined by means of temperature-dependent X-ray diffraction and TG-DSC. The first stage is characterised by the simultaneous formation of CeO2 and the cerous compound Ce(III)(2)(NH4)(3)(NO3)(9), isostructural with the compounds Ce(III)(2)M(I)(3)(NO3)(9)(M = K, Rb), which decomposes into CeO2 in the second stage. The process of the reduction of Ce(IV), induced by thermal decomposition, is discussed.