Three-dimensional numerical simulations of the reacting flow in rectangular microchannel PROX reactors are performed. To solve the set of governing equations, a finite volume method is applied using an improved SIMPLE algorithm. A three-step surface kinetics for the chemical reactions is utilized that includes hydrogen oxidation, carbon monoxide oxidation, and water-gas shift reaction. The kinetics chosen are for a Pt-Fe/gamma-Al(2)O(3) catalyst and operating temperatures of about 100 degrees C. The PROX reactor is expected to remove the carbon monoxide content in a hydrogen-rich stream from about 2% to less than 10 ppm. Effects of the inlet steam content, oxygen to carbon monoxide ratio, reactor wall temperature, aspect ratio of the channel cross section, and the channel hydraulic diameter are investigated. It is found that increasing the steam content, oxygen to carbon monoxide ratio, or wall temperature may improve the performance of the microreactor. it is also shown that the rate of water-gas shift reaction or its reverse is much lower than the oxidation reactions. Finally, it is revealed that based on a modified CO yield definition, the optimum channel geometry is a square shape. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.