CO oxidation on 2.2 +/- 0.5 nm platinum nanoparticles deposited on carbon with different platinum loadings (from 10% to 50%) has been studied and compared to the behavior of unsupported platinum nanoparticles. The different samples contain always nanoparticles of the same size and surface structure, and therefore the only characterization parameter that changes is the loading. TEM images and the voltammetric behavior in the supporting electrolyte demonstrate that the increasing loading leads to nanoparticle agglomeration, but the nanoparticles retain most of their individual physical identity and significant loss of active surface area does not take place. For the CO oxidation, the voltammograms show a single peak at ca. 0.8 V for the samples with low loading. As the loading increases, a second peak at lower potentials starts to develop and for the 50% loading sample and the unsupported nanoparticles, the peak is well resolved. Chronoamperometric transients also show the same behavior. The analysis of the results suggests that the peak at lower potentials (at shorter times in the transients) is associated to CO inter-particle oxidation, that is, OH and CO species that participate in the process are adsorbed in two different but close nanoparticles. The kinetic analysis of the two processes show different Tafel slopes, which indicates that the adsorption behavior of the OH species involved in the two peaks are different. (C) 2009 Published by Elsevier B.V.