Carbon monoxide is known to be poisonous to the proton exchange membrane fuel cell catalyst. Selective oxidation of carbon monoxide in a hydrogen-rich reformate stream is considered to be a practical method with the most potential for reducing concentrations down to tolerant levels. In the present work, nine different noble metal catalysts were investigated using a microstructured reactor in the presence of excess hydrogen and carbon dioxide at a GHSV of 15 500 h-1 and at temperatures up to 160 °C. The most active were Pt–Ru/¿-Al2O3, Rh/¿-Al2O3 and Pt–Rh/¿-Al2O3 yet the most stable was Pt–Rh/¿-Al2O3. Its activity was also investigated using a wet feed and also at a GHSV of 31 000 h-1. Water was found to promote the catalyst activity while at higher GHSV higher temperatures were required to achieve full carbon monoxide conversion. The catalyst exhibited steady performance in the microstructured reactor for 50 h while reducing 1.12% carbon monoxide to 10 ppm with inlet oxygen to carbon monoxide ratio of 4.