An artifact in the use of grab sampling techniques for the measurement of N2O emissions from fossil fuel combustion has been identified. Storing combustion products containing SO2, NOX and water for periods as short as two hours can lead to the formation of several hundred parts per million of N2O where none originally existed. The amount of N2O formed depends both on the amount of NOX and SO2 originally in the container. The experimental results are supported by a proposed chemical mechanism and kinetic calculations. An investigation of sampling protocols indicate that drying the gas to a 0°C dewpoint before introduction in the container reduces, but does not eliminate, N2O formation. More complete drying may yield a valid sample although this approach has, as yet, not been evaluated. Valid grab samples can be obtained by either 1) removing the SO2 before introducing the gas into the container; or 2) increasing the pH of the aqueous phase in the container with NaOH. The findings of this work have importa...