This work describes the thermal decomposition behaviour of cerium oxy-carbonate. DTA, TG, DTG and X-ray examinations were performed. The results indicate that the thermal decomposition of cerium oxy-carbonate in static air at a heating rate of 10 deg min-1 involves three steps: dehydration at 58 °C, removal of crystal water at 200 oC and CO2 release at 235 oC. The activation energies of removal of crystal water and carbon dioxide release were found to be 59.48 and 82.33 kJ mol-1, respectively. Ceria-based oxygen ion conductors have high ionic conductivity, which makes them interesting materials for application as solid electrolytes at moderate temperature ( < 600°), oxygen sensors and fuel cells [1]. The powder preparation is an important step in the fabrication of homogeneous materials. Chemical coprecipitation has been used. The properties of the final product (the sintered material or a mixed oxide powder) are largely dependent on the precipitation conditions. Precipitation as oxalate followed by calcination has been used to prepare doped oxide powders. This work describes a study of the thermal analysis and X-ray diffraction of cerium oxy-carbonate, which was prepared by precipitation from cerium nitrate solution with ammonium carbonate in the presence of ammonium hydroxide [2].