A number of different noble metal catalysts were studied for activity in the preferential oxidation of carbon monoxide in hydro.-en-rich streams. The prepared catalysts had an active phase of platinum, palladium, iridium, ruthenium Or rhodium impregnated either directly on an aluminia matrix or mounted on nickel or cobalt oxide. The catalysts were studied for activity, but also for the reversed water gas shift side reaction. The most active catalyst was platinum mounted on cobalt oxide and it was studied closely. The platinum on the cobalt oxide was studied using STEM with XEDS and the effects of the platinum cluster size were investigated. Via microscopy it was possible to show that platinum had no preference for either alumina or cobalt. The activity increased with increasing platinum cluster size but the selectivity passed through a maximum, a probable deactivation mechanism to the catalyst on stream, showed on a 1,000h deactivated sample. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.