Vibrational insights into the CO adsorption states on the (2 × 2) surface of the Pt3Sn(111) bulk alloy arereported for the first time by a systematic comparison between high-resolution electron energy loss spectroscopy(HREELS) measurements and density functional theory (DFT) anharmonic frequency calculations. CO adsorbspreferentially in top configuration on Pt atoms. In contrast with Pt(111), CO multifold sites are observedonly at low coverage. This difference is interpreted on the basis of the relative stability between top andthreefold hollow sites on Pt3Sn(111) compared to Pt(111). The evolution of the CO vibrational bands as afunction of temperature shows that CO adsorption is weakened by the presence of tin; this result is confirmedby DFT calculations.