The Au particle size dependence of CO oxidation on Au/CeO2 catalysts was extensively investigated. By keeping the size of CeO2 particle to be about 12 nm, the increasing size of Au particle from 3.9 to 7.5 nm caused significant decrease in the intrinsic activity for CO oxidation. Comparative study on sodium cyanide leaching, which effectively removed the metallic gold particles but retained the cationic gold clusters in the matrix of ceria, revealed that the cationic gold species could catalyze CO oxidation only at higher temperatures, confirming the major contribution of the metallic gold particles in the Au/CeO2 system. Therefore, it is evident that the Au-CeO2 interface determined mainly by the size of Au particle plays an essential role in achieving the high reactivity. (C) 2011 Elsevier B.V. All rights reserved.