The thermal decomposition of pure and rhodium impregnated cerium(III) carbonate hydrate in oxidising, reducing and inert atmospheres has been studied using combined thermogravimetry/mass spectrometry (TG/MS) and X-ray photoelectron spectroscopy (XPS). In oxygen, the decomposition of the pure carbonate proceeds in two steps, yielding H2O, CO2, and cerium(IV) oxide. In inert or reducing atmospheres, the second decomposition step is shifted towards higher temperatures and is divided into two parts. In the second part, CO2 evolved is partly reduced by Ce(III) to CO and to elemental carbon, and non-stoichiometric CeO2-x, is formed as the solid product. In the presence of rhodium as a reduction catalyst, decomposition in helium yields hydrogen and less carbon monoxide than that of the pure carbonate, due to the water-gas shift activity of the solid. In hydrogen, quantitative reduction of the carbon dioxide evolved to methane and water is observed when rhodium is present.