A high-temperature, stabilized-zirconia cell with an iridium cathode was used to electrochemically reduce low concentrations of nitric oxide (230-610 ppm) in streams containing oxygen (129-9800 ppm). A combination of thermodynamic considerations, responses to changes in the gas stream, and steady-state galvanostatic experiments suggest that the iridium electrode is converted to iridium dioxide by nitric oxide at open circuit and potentials more positive than -300 mV versus an air electrode. If the electrode is reduced to iridium metal at more negative potentials, nitric oxide can be electrochemically reduced more rapidly than oxygen.