This contribution aims at rationalizing the observations made by in situ IR spectroscopy during CO adsorption over 1.8 nm Pt-Sn nanoparticles by Density Functional Theory (DFT) calculations and describing the environment and electronic properties of surface Pt atoms modified by Sn. Pt surface enrichment upon CO exposure was observed by DRIFTS (diffuse reflectance FT-IR spectroscopy) and rationalized by the theoretical calculations, which also indicated that isolated Pt was favored over Pt pairing in a Sn-rich alloy. DFT frequency calculations allowing the fine assignment of vibrational CO spectra. The results show gradual