NaY-supported bimetallic Pt-Pd clusters have been studied by diffuse reflectance infrared spectroscopy of adsorbed CO. Spectra are quite different for Pt and Pd samples : on metallic Pt particles two singleton frequencies of linearly adsorbed CO molecules are observed whereas CO is mainly adsorbed on Pd in the form of bridging species. The spectrum of CO adsorbed on a bimetallic Pt-Pd catalyst is more complicated and suggests tile formation of Pt-Pd alloy particles having a Pd-enriched surface.