The kinetics of CO oxidation, H-2 oxidation and preferential CO oxidation (PrOx) over Au/Al2O3 catalysts have been investigated. The catalysts with the smallest particles (similar to 2 nm) are the most active for all three reactions. As previously observed, the presence of H-2 greatly promotes CO oxidation, which becomes faster than CO-free H-2 oxidation at low temperature. From these results and on the basis of previous works, we propose a complete PrOx mechanism. The reaction involves Au-OOH, Au-OH and Au-H intermediates, also involved in H-2 oxidation, and benefits from the presence of low-coordination sites. (c) 2009 Elsevier Inc. All rights reserved.