In this work, Sm2O3 was synthesized from the thermal decomposition of SmOCl (P4/nmm). The process was studied by gravimetry under flowing air between 600 degrees C and 950 degrees C. Reactants and products were identified and analyzed by X-ray diffraction and energy dispersive spectroscopy. From the results, the stoichiometry of the reaction was obtained. Between 800 degrees C and 950 degrees C, the successive formation of both cubic Sm2O3 (Ia3) and monoclinic Sm2O3 (C2/m) phases was observed. Below 800 degrees C, no transformation of cubic Sm2O3 to monoclinic Sm2O3 was detected. Quantification of phases was made using the Rietveld Method. Combined with the analysis of the evolution of the microstructure by scanning electron microscopy, an elemental analysis of the kinetics of the reaction was made and a reaction scheme was proposed. (C) 2006 Elsevier Ltd. All rights reserved.