CO poisoning of ethylene hydrogenation was studied on platinum and rhodium single crystals as well as on platinum nanoparticles deposited on alumina in the mTorr and Torr pressure regimes. Reaction studies using gas chromatography on Pt(111) show that CO poisons the reaction, and the measured activation energy in the presence of CO (20.2 kcal/mol) is higher than without CO (9.6 kcal/mol). STM studies on Rh(111) show that in the absence of CO, hydrogen and ethylidyne species that are present on the surface in large concentrations diffuse rapidly on the surface and thus, cannot be imaged. When CO is introduced, ordered structures appear on the surface. Based on these results, a model is proposed for CO poisoning on single crystals in which CO adsorbs on vacant hollow sites, preventing the diffusion of ethylidyne. With the immobile adsorbates filling the surface, ethylene from the gas phase has no room to adsorb, and ethylene hydrogenation is prevented. Similarly, CO on platinum nanoparticles reduces the reaction rate of ethylene hydrogenation. Unlike on Pt(111), however, CO does not change the activation energy significantly. This indicates that platinum at the oxide-metal interface sites remains active as CO is rapidly hydrogenated and removed from these sites. (C) 2003 Elsevier B.V. All rights reserved.