This invention represents a new class of microreactors. Microchannel reactors have been proven very effective in providing rapid heat transfer to support highly exothermic and endothermic reactions, such as steam reforming. Typically, these reactors use an interleaved architecture where they are for heat transfer between the reaction channels and heat exchange channels is comparable to the area for mass transfer from the reaction flow channel to the adjacent catalyst structure. This invention is useful for classes of reactions that are either mildly exothermic or endothermic, utilize a catalyst with low or moderate activity, or require carefully controlled heat transfer, such as when establishing a temperature profiles down the length of the reactor. In these cases, the normal interleaved approach can lead to an excessive area for heat transfer relative to the are for mass transfer, resulting in an oversized reactor, an unnecessary small temperature driving force, and/or quenching of the ration. This invention allows for much higher mass transfer area relative to heat transfer area to overcome these limitations. A prototype water-gas-shift reactor has been designed utilizing this concept, including detailed heat transfer calculations. A finite element simulation of this reactor is anticipated in the next several weeks. A prototype reactor will be built and tested over the next several months. The reactor will include a section that will be operated with a temperature profile down the length between 410 degrees C and 275 degrees C to implement the differential temperature concept. Implementing this invention for the WGS reactor of a steam reforming fuel processor for automotive fuel cell power system dramatically reduces the size and weight of this component enabling the system to approach power density and specific power targets for the Freedom Car. In addition, this invention provides improved catalyst productivity, reducing the catalyst cost