Reflection absorption infrared spectroscopy (RAIRS) has been used to investigate the adsorption of CO on CeO 2−x -supported Pd nanoparticles at room temperature. The results show that when CeO 2−x is initially grown on Pt(111), a small proportion of the surface remains as bare Pt sites. However, when Pd is deposited onto CeO 2−x /Pt(111), most of the Pd grows directly on top of the CeO 2−x (111). RAIR spectra of CO adsorption on 1ML Pd/CeO 2−x /Pt(111) show a broad CO–Pd band, which is inconsistent with a single crystal Pd surface. However, the 5ML and 10ML Pd/CeO 2−x /Pt(111) spectra show vibrational bands consistent with the presence of Pd(111) and (100) faces, suggesting the growth of Pd nanostructures with well defined facets.