Au-25(SPh-PNH2)(17) clusters deposited on mesoporous silica SBA-15 (mean particle size 0.9 +/- 0.2 nm) have been used as catalysts in the oxidation of benzyl alcohol. In the absence of radicals, fresh thiolate clusters are not active even in the presence of caesium carbonate. The activity strongly increases with calcination temperature and reaches a maximum when all ligands have been removed to form gold nanoparticles of ca. 1.8 nm. In the presence of radical initiators, functionalized clusters deposited on SBA-15 are very active, but the benzaldehyde formed is rapidly consumed by over-oxidation. The activity is attributed to the presence of -S-Au-I-S-bridges at the surface of the clusters. Upon calcination, the activity decreases due to reduction of Au-I species and the absence of radicals in the reaction medium. However, approximately 45% of alcohol is converted into benzaldehyde, with a selectivity of ca. 65% after 30 h. (C) 2014 Elsevier Inc. All rights reserved.