The objective of this report was to develop instruments for detecting CO, no, and NO2 in a mine atmosphere using fuel cell technology. The approach was to optimize the fuel cell sensor configuration for the specific detection of CO, no, and NO2 in the presence of other mine gases utilizing recent advances in catalyst, electrode, and solid polymer electrolyte membrane technology. Parametric testing was conducted on three electrode sensor cells to define performance and life characteristics as a function of temperature, feed flow, and applied sensor voltage. Extended life testing was performed on sensor cell configurations exhibiting favorable characteristics. Reliable, efficient gas sampling and electronic control circuitry was fabricated for use with the sensor cells and integrated into devices to meet the performance and design specifications for portable, battery-operated, hand-held direct- indicating gas detectors for use underground.