Ru-modified Pt nanoparticle surfaces were prepared using Ru electrochemical or spontaneous deposition on commercial-grade carbon-supported Pt nanoparticles (Pt-Vulcan XC72, E-TEK). Evidence for CO mobility was provided by cyclic voltammetry and FTIR reflectance spectroscopy experiments in weakly specifically adsorbing electrolyte such as 0.1 M HClO₄. Since the diffusion of CO is slow, a “two peak” voltammetric behavior was observed for 0 < θ_Ru < 10%. A further increase in Ru coverage leads to the observation of only one CO oxidation peak at a potential ca. 150 mV less positive. FTIR reflectance spectra recorded on Ru-modified Pt nanoparticles show a band located around 2000-2020 cm^-1, which is attributed to adsorbed CO on Ru sites. Exchange between CO species adsorbed on Ru and Pt sites was also observed along with a complete oxidation of a pre-adsorbed CO monolayer at a potential relevant for DMFC applications. These results confirm that the formation of a PtRu alloy is not a pre-requisite for obtaining CO-tolerant electrocatalysts.