Using high-resolution transmission electron microscopy (TEM), infrared reflection-absorption spectroscopy (IRAS), and electrochemical (EC) measurements, platinum nanoparticles ranging in size from 1 to 30 nm are characterized and their catalytic activity for CO electrooxiclation is evaluated. TEM analysis reveals that Pt crystallites are not perfect cubooctahedrons, and that large particles have "rougher" surfaces than small particles, which have some fairly smooth (111) facets. The importance of "defect" sites for the catalytic properties of nanoparticles is probed in IRAS experiments by monitoring how the vibrational frequencies of atop CO (v(CO)) as well as the concomitant development of dissolved CO2 are affected by the number of defects on the Pt nanoparticles. It is found that defects play a significant role in CID "clustering" on nanoparticles, causing CO to decrease/increase in local coverage, which yields to anomalous redshift/ blueshift v(CO) frequency deviations from the normal Stark-tuning behavior. The observed deviations are accompanied by CO2 production, which increases by increasing the number of defects on the nanoparticles, that is. 1 &LE; 2 < 5 &MLT; 30 nm. We suggest that the catalytic activity for CO adlayer oxidation is predominantly influenced by the ability of the surface to dissociate water and to form OHad on defect sites rather than by CO energetics. These results are complemented by chronoamperometric and rotating disk electrode (RDE) data. In contrast to CO stripping experiments, we found that in the backsweep of CO bulk oxidation, the activity increases with decreasing particle size, that is, with increasing oxophilicity of the particles.