Oxidation of CO on platinum nanoparticles ranging in size from 1 to 30 nm has been studied in acid electrolytes. We found that Pt nanoparticles, characterized by transmission electron microscopy, are not perfect cubo-octahedrons and that large particles have "rougher" surfaces than small particles. The importance of "defect" sites for the catalytic properties of nanoparticles was probed by using infrared reflection absorption spectroscopy (IRAs) and rotating disk electrode. From IRAs experiments, by monitoring how the vibrational frequency of a-top CO (v(CO)) as well as the concomitant development of dissolved CO2 are affected by the number of defects on Pt nanoparticles, we suggested that defects play a significant role in CO "clustering" on nanoparticles, causing CO to decrease/increase in local coverage, which results in anomalous redshift/blueshift v(CO) frequency deviations from the normal Stark-tuning behavior. The observed v(CO) deviations are accompanied by CO2 production, which increases by increasing the number of defects on the nanoparticles, i.e., 1 <= 2 < 5 << 30 nm. We suggest that the catalytic activity for CO adlayer oxidation (CO stripping) is predominantly influenced by the ability of the surface to dissociate water and to form OHad on defect sites. We demonstrate that the catalytic activity of Pt nanoparticles for CO oxidation under the condition of continuous CO supply to the surface depends on the pre-history of the electrode. If the surface is precovered by CO, the particle size has a negligible effect on CO oxidation. However, on an oxide-precovered surface CO bulk oxidation increases with decreasing particle size, i.e., with increasing oxophilicity of the particles. We found, if specific sites on the surface are active for OH adsorption, then the electrocatalytic activity for CO oxidation changes as the concentration of these sites changes with particle size. (C) 2005 Elsevier Ltd. All rights reserved.