It was disclosed for Au/zeolites that not one, but several active sites can be reactive in CO oxidation on the same catalyst. Au3+ cationic species do not show the activity under studied conditions. Gold nanoparticles (1.5-5 nm) are characterized by activity at high temperature. Gold clusters display activity in low-temperature region, but only after sample pretreatment in He at 500 degrees C. Nanoparticle active sites are stable during reaction, in contrast with cluster active sites. The last ones lost activity probably due to complete oxidation and can be reactivated by He treatment. This investigation revealed that clusters are the most active, but not stable and are easily oxidized by oxygen, while stable nanoparticles are less active. (C) 2007 Elsevier B.V. All rights reserved.