The Ni/CeO2 catalysts with successive oxidization, reduction and re-oxidization have been tested for hydrogen production in sulfur-iodine (SI or IS) cycle. The samples were characterized by BET, XRD, TEM, TPR and XPS. The oxidative/reductive atmosphere affected the structure and performance of the catalysts. It was suggested that a migration of Ce4+ from the bulk to the surface occurred during the reductive treatment. The diffusion process was reversed when the atmosphere was changed to an oxidative one. The reduced and reoxidized samples seemed to be similar all the time and showed better catalytic activity in comparison with the as-received and oxidized samples. For the re-oxidized sample, the strongest interaction compared with other samples occurred between Ni and CeO2 and oxygen vacancies transferred from bulk to surface, which led to form more surface sites and oxygen vacancies. The metal Ni was found only on the surface of the reduced sample. The active site of metal Ni besides the surface site and oxygen vacancy were assumed to play an important role for hydrogen production. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.