CO oxidation pathways over Au nano-clusters, such as face centered cubic Au-14,icosahedron Au-13,cubo-octahedron Au-13 and tetrahedron Au-20 were investigated by using PIO analysis proposed by Fujimoto et al. Four types of oxidation pathways were examined: reaction between (a) adsorbed CO and gaseous O-2 (E-R mechanism), (b) adsorbed O-2 and gaseous CO (E-R mechanism), (c) adsorbed CO and vicinally adsorbed O-2 (L-H mechanism), and (d) adsorbed CO and geminally adsorbed O-2 (L-H mechanism). Only in the case of L-H mechanism, when CO molecule and O-2 Molecule are adsorbed on adjacent two surface An atoms in vicinal manner (c), CO oxidation occurs favorably. In comparison of the vicinal CO and O-2 co-adsorption on icosahedron Au-13 with that on cubo-octahedron Au-13, CO oxidation is favorable in the former, whereas it is unfavorable in the latter because of considerably longer distance between CO and O-2. It is also predicted that CO oxidation is also favorable in the case of vicinally co-adsorbed state of CO and O-2 on tetrahedron Au-20. The present work has shown that the PIO analysis based on extended Huckel MO is an effective way to investigate reaction pathways, especially in large catalytic systems. (c) 2005 Elsevier B.V. All rights reserved.