The steam treatment effect has been investigated over the doubly impregnated catalyst, Ni/Ce-ZrO2/theta-(AlO3)-O-2, in steam methane reforming (SMR). The catalyst was remarkably deactivated by steam treatment but reversibly regenerated by H-2-reduction. XRD results showed that the steam treatment resulted in the formation of NiAl2O4 which is inactive for SMR but it was reversibly converted to Ni by the reduction. The reversible oxidation-reduction of Ni state was also evidenced by XPS and it was observed that the formation of NiAl2O4 is more favorable at higher temperature. It is most likely that the alumina support is only partially covered with Ce-ZrO2 and most Ni directly interacts with theta-Al2O3 which would probably make easy formation of NiAl2O4 in the presence of steam alone. The results imply that, during the start-up procedure in SMR, too high concentration of steam could deactivate seriously Al2O3 supported Ni catalysts. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.