This study focuses on clarifying the strong interaction existing between extended graphitic domains of ordered carbonaceous materials such as multiwalled carbon nanotubes and platinum nanoparticles. This interaction results from the heterogeneous nucleation of platinum nanoparticles onto the carbon support. The metal clusters are chemically synthesized by using the carbonyl route. Two different carbon supports are used namely, homemade multiwalled carbon nanotubes, MWCNT-m, and classical Vulcan XC-72. Physicochemical properties of these materials are described by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The effect of the strong interaction on the electronic properties of platinum nanoparticles is electrochemically probed by means of CO stripping experiments coupled with in situ Fourier transform infrared spectroscopy (FTIR). Density functional theory (DFT) is used to evaluate changes to the electronic structure of a platinum cluster interacting with a graphite substrate and their effects on CO adsorption on the cluster. Results are correlated with structural and electronic properties of platinum nanoparticles. The stability of Pt/carbon catalysts under electrochemical potential cycling is correlated with the properties of carbon substrates.