In this paper, we present experimental results on the design, scale-out, and operation of a preferential CO oxidation microdevice, consisting of a numbered-up array of 26 microchannel reactors running in parallel. In this design, the external surface area of the individual microchannel reactors is used for heat-exchange with the environment to maintain the desired operation temperature. Thus, the heat flux via the external surface areas of the parallel microchannel reactors and the fittings is equal to the heat producedby the reaction. While this design does not allow to recover the heat produced by the PrOx reaction, it is simpler and cheaper as compared with integrated microreactor/heat-exchanger configurations as proposed by Cominos et al. [12] and Delsman et al. [14,15].This paper presents different aspects of the CO PrOx microreactor assembly design as well as experimental results from catalyst testing