A Ru/SnO2 catalyst was prepared by capturing "unprotected" Ru nanoclusters on SnO2 colloidal particles via electrostatic interaction followed by gelating the obtained complex sol by adjusting its pH value to 7. The assembled Ru/Sn02 catalyst performed excellently in selectively hydrogenating ortho-chloronitrobenzene to the corresponding o-chloroaniline, the catalytic activity surpassing that of previously reported Ru catalysts and the selectivity for ortho-chloroaniline in agreement with the best results reported so far on a colloidal Ru catalyst. Obvious promotional effects of the SnO2 support on both the catalytic activity and the selectivity were observed for the first time. A decrease of the reactivity of the C-Cl bond of o-chloroaniline over the present catalyst is one of the predominant reasons for the high selectivity at complete conversion. The catalyst structure was characterized by STEM, XPS, BET, and XRD. (C) 2004 Elsevier Inc. All rights reserved.