The effect of internal mass transport limitation on the preferential CO oxidation in hydrogen-rich mixture over copper-cerium oxide catalyst in a form of pellets and washcoat in microchannel reactor is estimated. Internal effectiveness factor eta(CO) >0.8 in the optimum interval of reaction temperature (170-230 degrees C) is reached if the pellet diameter and washcoat thickness do not exceed 100 and 20 mu m, respectively. Compared to conventional packed-bed reactor with catalyst pellets, microchannel catalytic washcoated reactor is more appropriate for practical use. (C) 2011 Elsevier B.V. All rights reserved.