The synthesis and formation mechanism of the nano-sized Ce0.8Sm0.2O1.9 particles prepared by a urea-based low-temperature hydrothermal process was investigated in this study. From ex situ X-ray diffraction and induced coupled plasma-atomic emission spectroscopy investigations, it was found that large quantities of cerium hydroxide co-precipitated with some samarium hydroxide at the initial stage of the hydrothermal process. The remaining Sm3+ ions in the solutions were further hydrolyzed and deposited on the surface of the cerium hydroxide-rich precipitates to form a core-shell-like structure. During the hydrothermal process, the core-shell-like structure transformed to a single cubic fluorite phase which is due to the incorporation of the deposited samarium hydroxide into the cerium oxide-rich core. Further, the average grain size of the synthesized nanocrystalline Ce0.8Sm0.2O1.9 was reduced with increasing the urea concentration in the solution. The density of the disk prepared with the synthesized Ce0.8Sm0.2O1.9 powders was found to increase with a decrease in the grain size of Ce0.8Sm0.2O1.9. The existence Of SO42- anions in the SDC powders prepared at low-urea concentration may result in the SDC disks with low density due to their decomposition during sintering process. (c) 2007 Published by Elsevier B.V.