A silicon-based micro-reactor was fabricated using a micro-electromechanical system (MEMS) process and silicon micromachining technology (SMT) to allow preferential CO oxidation (PrOx) to be applied to compact portable devices. To design the micro-PrOx reactor, the effects of channel length. channel width, channel array. and O-2 content in the reactant gas on the CO conversion and selectivity for CO were investigated. The single micro-PrOx reactor required an O-2 concentration four times higher than the theoretical quantity for a complete conversion of 1% CO into CO2. The optimum operating temperature to obtain high CO conversion, by suppressing the reverse water-gas shift (r-WGS) reaction, depended considerably on the channel lengths of the micro-reactor rather than on the channel widths or the channel arrays. The micro-PrOx reactor showed 99.4% conversion of CO and 44.14% selectivity for CO at 260 C when operating with 1% CO, 50% H-2. 35% CO2, N-2 (balance), and 2% O-2. (c) 2008 Elsevier B.V. All rights reserved.