A new cerium(IV) oxide nitrate, (\mathrm{Ce}{2} \mathrm{O}\left(\mathrm{NO}{3}\right){6}\left(\mathrm{H}{2} \mathrm{O}\right){6} \cdot 2 \mathrm{H}{2} \mathrm{O}), was synthesized from the dissolution in nitric acid of hydrous ceria with high specific area. Its crystal structure was determined from three-dimensional X-ray diffraction data. Crystals are monoclinic, space group (P 2_{1} / c), with (a=8.7233(8), b=8.9397(8), c=) 13.981(1) (\AA, \beta=94.909(7)^{\circ}), and (Z=2). Full matrix least-squares techniques led to a conventional (R) factor of 0.023 for 2091 observed reflections. The structure consists of dimer units with formula (\mathrm{Ce}{2} \mathrm{O}\left(\mathrm{NO}{3}\right){6}\left(\mathrm{H}{2} \mathrm{O}\right){6}), which are formed by two equivalent 10 -fold cerium(IV) polyhedra connected by an oxo bridge. Additional water molecules are located between these units. Hydrogen bonds ensure the continuity of the structure. The thermal decomposition under vacuum of this compound was investigated by means of sequential thermodiffractometry. Two new phases, (\mathrm{Ce}{2}) (\mathrm{O}\left(\mathrm{NO}{3}\right){6} \cdot 3 \mathrm{H}{2} \mathrm{O}) and (\mathrm{Ce}{2} \mathrm{O}\left(\mathrm{NO}{3}\right){6}), were identified and finely dispersed (\mathrm{CeO}_{2}) powder was obtained at low temperature (\left(180^{\circ} \mathrm{C}\right)). The microstructural features of the oxide annealed at (600^{\circ} \mathrm{C}) were also studied, showing the "isotropic" shape of the crystallites. 1994 Academic Press, Inc.