In this work we investigate some aspects of the electrooxidation of CO in dispersed Pt nanoparticles. In order to verify the effect of the particle size on the kinetics of CO electrooxidation we investigated morphological characteristics of these materials as size distributions and agglomeration patterns. FTIRS in situ and DEMS were used to follow the oxidation of CO to CO2 as a function of the applied potential. The results are coherent with those previously found in the literature: CO oxidation is inhibited in the presence of small Pt particles. The results are rationalized in terms of the size-dependence diffusion coefficient for adsorbed CO which provokes a lower CO-mobility when small particles are used as catalysts. Additionally, multiple CO vibrational bands are visualized for the first time in Pt nanoparticles by using FTIRS in situ. Spectroscopic results show that CO2 formation occurs systematically earlier when the size of the Pt particles is increased. Moreover, dipole–dipole coupling is not so pronounced for small particles, allowing the discrimination of at least four CO distinct bands in the range comprised between 1845 and 2060cm−1.