An artifact in the use of sampling containers for the measurement of nitrous oxide emissions from fossil fuel combustion has been identified. The storage of moist combustion products containing SO2 and NO for periods as short as 2 hours can lead to the formation of several hundreds of parts-per-million of N2O in the sample containers where none originally existed. The drying of the gas in a 0°C trap before introduction into the container reduces, but does not eliminate N2O formation. Removing SO2 prior to the sample container appears to eliminate the artifact. This finding may have important consequences with respect to the validity of the existing data base on N2O emissions from combustion sources and the role of combustion generated N2O in the atmospheric N2O balance.