CO adsorption on AunY (n = 1-9) is investigated using density functional theory, and compared with pure gold clusters Aun+1. CO is confirmed to prefer on-top binding to the least coordinated Au atom for the pure gold clusters, whereas for Y-doped gold clusters, the most favorable sites are dependent on the cluster size; the Au on-top sites are favored from n = 6. CO behaves as an electron acceptor, and the C-O vibrational frequency of the metal-CO complexes correlates with the degree of charge transfer to CO. Reactivity with CO is decreased after one gold atom is replaced by one yttrium for most sizes. (C) 2010 Elsevier B.V. All rights reserved.