The electrochemical oxidation of carbon monoxide on a platinum–Nafion electrode was examined. It was found that the oxidation of CO on the Pt‐Nafion electrode is more favored in an acidic solution than in an alkaline medium. This is in contrast with what was reported for bulk metal electrodes, for which the oxidation rate of CO in acid is much smaller than in alkaline solutions. This phenomenon is believed to be directly related to the cation‐exchange capability of the Nafion membrane. The rest potential of the Pt–Nafion electrode was found to be dependent on the partial pressure of CO, which is also in contrast with the behavior of bulk metal electrodes. This result was attributed to a mixed potential phenomenon. A quantitative determination of CO concentration in the gas phase was carried out on the basis of the Pt–Nafion electrode system in a controlled‐potential mode. The results obtained showed a number of attractive properties, including a low detection limit, good reproducibility, and a fast response time.