Abstract Carbon monoxide adsorption has been studied on (111) faces of single-crystal alloys — Pt 10Ni 90, Pt 50Ni 50 and Pt 78Ni 22 — by vibrational EELS and TDS measurements. On the nickel rich sample, on which the outer layer is notably enriched in platinum with respect to the nominal bulk concentration, multi-bonded nickel sites and on-top platinum sites for CO chemisorption are evidenced. The multi-bonded sites are preferentially filled. In the platinum rich samples, which exhibit a quasi complete platinum surface layer, linearly bonded species are observed first; multi-bonded CO adspecies appear only at higher coverage. Whatever the considered alloy sample the metal-CO bond strengths are weaker than on pure metals, as demonstrated by the lower values of both the metal-CO stretching mode energy and the temperature of the thermodesorption mass-28 peak. That weakening is more pronounced on the Pt 50Ni 50 sample.