Density functional theory calculations were carried out for CO upside-down adsorption, namely the O atom close to the metal atom, on small PtmAun clusters (m, n = 0-4) and on Pt(111), Au(111), and Pt0.25Au0.75(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 PtmAun clusters may be detected using FTIR coupled with TPD techniques. (c) 2007 Elsevier B.V. All rights reserved.