A thin alumina film, the alumina model surfaces modified by the metallic molybdenum and by the MoO3 are titrated by CO chemisorption. Two types of CO adsorption sites, namely octahedrally and tetrahedrally coordinated Al3+ Sites, are present on the thin alumina film. The thin alumina film prepared under UHV conditions can be used to simulate the conventional high-surface-area alumina supports in real catalysis. The deposited metallic molybdenum preferentially occupies octahedrally coordinated Al3+ site and suppresses CO chemisorption on this site, and oxidation of the surface molybdenum species enhances this suppression. (c) 2007 Published by Elsevier B.V.