Density functional theory calculations were carried out for CO upside-down adsorption, namely the O atom close to the metal atom, on small Pt m Au n clusters (m, n=0–4) and on Pt(111), Au(111), and Pt 0.25 Au 0.75 (111) surfaces. The results show that CO can adsorb upside-down on the clusters but not on the bulk surfaces. Analysis combining both the adsorption energy and CO stretching frequency data indicates that the CO upside-down adsorption on pure Pt clusters may be detected experimentally using FTIR spectroscopy alone while those on pure Au and Pt m Au n clusters may be detected using FTIR coupled with TPD techniques.