The electrocatalytic properties of home-made Pt nanoparticles supported onto WO_x were determined for the electrooxidation of a CO_ads monolayer and compared with that of a commercial Pt/C having the same Pt particle size. By combining electrochemical and spectroscopic techniques, we found that Pt/WO_x nanoparticles exhibit a very high tolerance to CO at low electrode potentials (E = 0.1 V vs. RHE), which was never reported in the literature before. CO adsorption at E = 0.1 V vs. RHE on Pt/WO_x yields CO₂ production as observed by Fourier-transform infrared spectroscopy (FTIR). When the gas bubbling in solution changes from CO to Ar, the current attenuates and the CO₂ production vanishes. This points towards a limited number of “active sites” and a slow step in the electrocatalytic process. When H₂ is used to purge the electrolyte from CO, a steep and continuous increase of the H₂ electrooxidation current is observed pointing towards continuous liberation of the Pt catalytic sites. The high tolerance to CO of Pt/WO_x is discussed in terms of strong metal–support interaction (SMSI), which involves formation of a metal-oxide film partially covering the Pt nanoparticles (encapsulation) and creation of W–OH groups upon H⁺ insertion at low electrode potentials.