Nanoscale materials have attracted great interest because of their distinct properties. By means of XPS, the present work investigated the difference of reduction behavior between nanocrystalline and microcrystalline ceria on condition of Ar+ bombardment or X-ray irradiation. For the first time, the results indicate that the reduction level of Ce4+ to Ce3+ is lower for nanocrystalline ceria than for microcrystalline ceria although the experimental conditions are identical. These differences have been attributed to the differences in the concentration of oxygen vacancies in the bulk and the diffusion ability of oxygen atoms between them. Besides, the key factor for the reduction of ceria produced by X-ray exposure is discussed. (c) 2005 Elsevier B.V. All rights reserved.