The following paper shows the successful application of some selective oxidation (selox) and water gas shift (WGS) catalysts to microreactors, which allow to reduce the CO content in a model gas mixture that contains the major product components of a reformer or of partial oxidizer used for hydrogen production for fuel cells. At small average residence times of less than 30 ms, which are typical for microstructured reactors, the water gas shift reaction at 250-300 degreesC using a Ru/ZrO2 catalyst allowed to reduce the CO content by more than 95%. By means of the selox reaction, a residual CO content of 1 vol.% could be oxidized to CO2 with a conversion of more than 99% when using a CuO/CeO2 catalyst at less than 150degreesC and an average residence time of 14 ms. However, CO-selectivity amounted to 20% for the selox reaction only. For both the WGS and the selox reaction, those catalysts that have the largest surface enlargement are the most active ones. The prepared selox catalysts allow the temperature range of selective CO oxidation to be extended considerably. The usable temperature range is increased from 60-120degreesC (conventional reactors) to 60-180degreesC. Because of the small dimension and the high heat transfer coefficients of a microreactor system, temperatures can be reached very fast. Additional temperature ranges can be controlled exactly, which is important to maximize the CO-selectivity. (C) 2003 Elsevier B.V. All rights reserved.