The production of hydrogen from synthesis gas with a high CO content is investigated with microstructured reactors. The main advantages of using microstructured reactors are the excellent heat and mass transfer characteristics inside of the reactor and the possibility to include heat exchangers to reduce the system size and heat losses. High- and low-temperature water gas shift reactions are examined for the ability to decrease the CO fraction of the synthesis gas down to a level tolerable for proton exchange membrane fuel cells. These gas purification methods have the advantage of a higher hydrogen yield being achievable by '' converting '' CO into H-2, thus leading to a higher overall efficiency of the fuel cell system. A next step in further reducing the CO content could beif necessarypreferential oxidation. The high-temperature shift reaction takes place at a temperature of approximately 350 degrees C in the presence of an iron-or chromium-based catalyst. The low-temperature shift reaction is operated at approximately 250 degrees C with copper catalysts. The performance of microstructured reactors fitted with different catalysts is discussed within this paper as a function of the temperature, gas flow rate, and gas composition.