The thermal decomposition process of Ce(NH4)(2)(NO3)(5) . 4H(2)O has been determined by means of temperature-dependent Xray diffraction and TG-DSC. The transformations are particle-size dependent and have been studied in nitrogen and vacuum. The occurrence of Ce(NH4)(2)(NO3)(5) . 3H(2)O, Ce(NH4)(2)(NO3)(5) . 2H(2)O, Ce(NH4)(2)(NO3)(5), Ce-2(NH4)(3)(NO3)(9) or a mixture of these last two phases, as well as the formation of amorphous NH4NO3 have been demonstrated. The diffraction powder patterns have been indexed, a linear variation of the chemical-formula-unit equivalent volumes V-eq with the number of water molecules has been derived from the unit-cell volumes.