Identification of the active sites is a key factor in understanding the mechanism of gold-catalyzed reactions. In this work we investigated the CO adsorption properties of gold-based model catalysts ranging from native and surface-modified single crystal surfaces to polycrystalline thin films promoted by iron oxide layers. The results, completed with those obtained on a gold nanoparticle-containing system, clearly demonstrate the role of the defect sites in the CO adsorption. In addition, a promoter such as iron oxide can further enhance CO adsorption, probably through a mechanism connected to the interface formation between gold and iron oxide. (C) 2010 Published by Elsevier B.V.