In this work, ceria (CeO(2)) prepared with different methods and at various calcination temperatures have been tested to evaluate their effect on hydrogen iodide (HI) decomposition in sulfur-iodine (SI) cycle at various temperatures. The CeO(2) catalysts' strongly enhance the HI decomposition by comparison with blank test, especially gel CeO(2) 300. TG-FTIR, BET, XRD, TEM and TPR were performed for catalysts' characterization. The results show that the CeO(2) catalyst synthesized by citric-aided sol-gel method and calcined at low temperature (<500 degrees C) shows more lattice defects, smaller crystallites, larger surface area and better reducibility. oxygen can promote the significantly rapid surface reaction, but simultaneously consume hydrogen species (H) contained in HI. Lattice defects, especially the reduced surface sites, i.e., Ce(3+) and oxygen vacancy, play the dominant role in surface reactions of HI decomposition. A new reaction mechanism for HI catalytic decomposition over CeO(2) catalyst is proposed. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.