A novel structured catalyst made up of mesoporous Co(3)O(4) nanowire arrays (active phase) supported on a stainless steel mesh is presented. An important advantage of this catalytic system is that the formation and growth of cobalt oxide nanowires takes place on the surface of stainless steel wires. These mesoporous Co(3)O(4) nanowires have a diameter of 500 nm, a BET surface area of 71 m(2)/g and a pore size of similar to 3.4 nm. The catalytic behavior of this system towards the preferential oxidation of CO was investigated. To this end a microreactor was constructed consisting of a metal wire mesh-Co(3)O(4) nanowire composite rolled and inserted into a standard 1/4 '' stainless steel tube. This structured catalytic system exhibits several important properties: a) a high heat exchange rate and a low pressure drop, b) a remarkable stability over the whole temperature range analyzed (100-175 degrees C) and c) a good catalytic activity which doubles that of the best Co(3)O(4) catalyst previously reported. An added feature of this synthetic procedure is that it allows the simultaneous synthesis of unsupported porous Co(3)O(4) particles (similar to 100 mu m diameter) with slightly different characteristics to those of the supported nanowires. Catalytic tests on these particles highlighted the close relationship between the calcination temperature, the reducibility of the produced spinel and the catalytic activity during the PROX reaction. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.