CO adsorption onto Pt- and Pt-based catalysts is a very relevant topic in electrocatalysis, and in particular, in fuel cells research. CO is known to be the main responsible of the poisoning of the catalysts and consequent decrease on performance of the fuel cells devices. In this paper, density functional theory (DFT) calculations and experiments were combined to access the effect of modifying Pt nanoparticles with Bi on CO adsorption. CO adsorption energies were calculated for Pt sites nearby Bi atoms in different type of structures: Pt clusters with Bi as surface adatom and Pt clusters with Bi as surface and subsurface dopant (alloys). The results show that, when compared with pure Pt, the adsorption energies for CO are lower on PtBi clusters in both adatom and surface alloy configurations. Subsurface PtBi alloys reveal higher adsorption energies for CO but these structures are energetically very unfavorable. On the basis of the calculations, a high degree of mobility of Bi on the surface was found in the presence of CO. These results suggest that the experimental differences between cyclic voltammogram before and after CO stripping can be due to a reorganization of the Bi layer on the catalyst when CO is coadsorbed. It was also experimentally observed, that CO oxidation peaks on the modified electrodes shift to higher potentials with increasing Bi coverage. These results suggest a higher effect of the decrease of CO coverages on the oxidation process than the decrease of the biding energies for Pt-CO in the presence of Bi.