The electrooxidation of CO on Ru(0 0 0 1) and RuO2(1 0 0) electrode surfaces were characterized by cyclic voltammetry, Auger spectroscopy (AES) and reflection high-energy electron diffraction (RHEED), For CO saturated Ru(0 0 0 1) surface, the present CO oxidation processes result from direct attack of oxygen species and slow erosion, which are controlled by oxygen diffusion from the near-surface region toward the surface. In contrast to Ru(0 0 0 1) surface, high activity of RuO2(1 0 0) towards CO oxidation was demonstrated, the CO molecules can directly react with two different lattice-oxygen on the surface to carbon dioxide. (C) 2002 Elsevier Science B.V. All rights reserved.