In this study a series of CeO2 powders with surface area in the range of 10-140 m(2)/g were prepared by calcination at increasing temperatures of samples of polycrystalline ceria and used as catalysts for CO oxidation under a free diffusion limitation regime. It was found that aging under air of polycrystalline ceria samples, regardless of the preparation method used, caused the rearrangement of crystallites, with loss of surface area and exposure of more reactive surfaces, which increases the specific activity in CO oxidation. HRTEM analysis has shown that the treatment under oxygen at temperatures greater than 773 K results in the truncation of the sharp tips between two {111}-type surfaces, exposing new and more reactive surfaces of the I 100 1 type. In accordance with previous theoretical calculations and with recent studies on monodimensional ceria samples, a correlation between exposure of these surfaces and an increase in specific reaction rate was found. (c) 2005 Elsevier Inc. All rights reserved.