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We present a suspended-tube chemical reactor/heat exchanger for high-temperature fuel processing in micro energy conversion systems, primarily for hydrogen production in portable fuel cell systems. This reactor, designed to thermally isolate a high-temperature reaction zone, consists of four free-standing silicon nitride tubes comprising two independent U-shaped fluidic channels. Portions of the tubes are encased in silicon to enable heat exchange between the fluids in these channels. A thin-film platinum resistor is embedded for localized heating and temperature sensing. This paper describes the design and fabrication process for the MEMS fuel processor. Fluidic testing, thermal characterization up to 825°C, and preparation of catalyst washcoats in the reactor microchannels are discussed. In addition, results from catalytic autothermal butane combustion and ammonia cracking in the reactor are presented.