In this paper, we use Fourier transform infrared (FTIR) spectroscopy and stripping voltammetry at saturation and submonolayer CO coverages to shed light on the influence of size on the CO adsorption and electro-oxidation on Pt nanoparticles. Pt nanoparticles supported on low surface area (similar to1 m(2) g(-1)) carbon (Sibunit) are used throughout the study. The vibrational spectra of adsorbed CO are dominated by interparticle heterogeneity (contribution of particles of different size in the range from 0.5 to 5 nm) rather than intraparticle heterogeneity (contribution of different adsorption sites). CO stripping voltammetry exhibits two peaks separated by approximately 0.25 V (at 0.02 V s(-1)), which are attributed to the CO oxidation from "large" (similar to3.6 nm) and "small" (similar to1.7 nm) Pt nanoparticles. Using stepwise oxidation, we were able to separate the contributions of "large" and "small" nanoparticles and obtain their infrared and voltammetric "fingerprints". Considerable differences are observed between "large" and "small" nanoparticles in terms of (i) the vibrational frequencies of adsorbed CO molecules (ii) their vibrational coupling, and (iii) CO oxidation overpotential.