A sensor to detect the carbon monoxide in a reformate stream that fuels a polymer electrolyte membrane (PEM) fuel cell is presented. This electrochemical sensor consists of a Nafion(R) electrolyte and Pt- or Ru-based electrodes and works on the principle of differential CO poisoning of the various precious metal electrodes. Varying the composition and loading of the precious metal in the electrode optimized the response of the sensor. A sensor with a working electrode of Pt and a pseudo-reference electrode of Pt-Ru alloy and a precious metal loading of 10 mg/cm(2) showed a stable and reproducible response to 10-200 ppm of CO at room temperature and 100-1000 ppm CO at 70 degreesC. (C) 2004 Elsevier B.V. All rights reserved.