The electron emission characteristics of Spindt-type platinum field emitters that were improved by operation in a carbon monoxide gas ambient were investigated in detail. We present differences in the emission characteristics, such as the change of Fowler-Nordheim characteristics in the S-K chart, change in current fluctuation and emission pattern, and resistance against oxygen exposure due to the improvement. We found that the work function increased slightly during the introduction of carbon monoxide gas up to 10(-4) Pa; but when the gas was evacuated, the work function recovered. The work function finally decreased at a pressure of 10(-3) Pa. The emission characteristics were degraded during the oxygen exposure both for the nontreated and treated emitters. However, degradation was less significant for the carbon-monoxide-treated emitter than for the nontreated emitter. All these results suggest that the improvement can be attributed to the presence of carbon atoms or a carbon layer at the surface of the emitter.