Nanometer-sized gold particles on oxide supports are efficient catalysts for the selective catalytic oxidation (SCO) of carbon monoxide under conditions compatible with the operation of PEM fuel cells. Nanosized Au/␥-Al 2 O 3 catalysts are able to oxidize CO between 20 and 70°C in an atmosphere of hydrogen combining high CO conversion and satisfactory selectivity to CO 2 (1). It is generally agreed that the catalytic activity of gold depends on the size of the gold particles, but the nature of support material, the preparation method, the activation procedure have also been suggested to play a key role (2). Sites at the gold support interface have also been claimed to be responsible for the activity in CO oxidation (3). Strain in the Au particles due to the mismatch of the lattices at the interface with the support (4) and the effect of low-coordinated sites and roughness (4) have also been suggested as important factors for high activity. The most important effect, concerning the catalytic activity of gold nanoparticles for low temperature oxidation of CO is related to the availability of many low-coordinated gold atoms on the small particles (5). Effects related to the interaction with the support may also contribute, but to a considerably smaller extent (5).CO oxidation over group VIII noble metals is the most studied catalytic reaction. However, selective oxidation of CO in hydrogen-rich conditions is not as well studied (6,7). CO (reactant) and CO 2 (product) sorption processes are fundamental elementary steps for SCO and moreover, the effect of hydrogen on their sorption over supported Au catalysts remains a subject of significant interest.Among the various supported Au catalysts, Au/Al 2 O 3 is perhaps one that has shown the widest variation for CO oxidation, ranging from being very inactive to practically as active as Au/TiO 2 (8). Compared to Au/␥-Al 2 O 3 , multicomponent gold-based catalysts also supported on ␥-Al 2 O 3 and combined with common metal oxides, such as: MnO x , MgO, FeO x (9,10) as well as gold catalysts supported on other materials such as Au/TiO 2 (11) and Au/FeO x (11,12) have been found to exhibit even more promising commercial performance for the SCO reaction. However, based on its simplicity, a well-studied Au/␥-Al 2 O 3 catalyst (1,9) has been utilized in the present work as model system, in order to gain fundamental information on the catalytic behaviour of gold nanoparticles and identify the active sites on Au/␥-Al 2 O 3 .In this paper, new findings which offer further information concerning the mechanism of CO preferential oxidation over nanosized Au are presented. The first finding concerns CO 2 formation in the absence of oxygen and hydrogen both in bare ␥-Al 2 O 3 as well as in Au/␥-Al 2 O 3 (called as CO decomposition). At high temperatures (>200 °C) in excess of H 2 , over the Au/␥-Al 2 O 3 catalyst, reversed water gas shift (RWGS) reaction results in CO 2 consumption towards CO and H 2 O formation. Finally, the kinetic measurements of the present work indicate that hydrog...