Apparatus has been developed for the mass spectrographic study of ion‐molecule reactions occurring under gas‐kinetic conditions. Ions are produced by electron bombardment inside a long drift tube containing gas at a pressure of up to about 0.7 torr. The ions diffuse down the drift tube under the influence of a weak electric field. A sample of the ion population at the end of the drift tube is extracted through a two‐stage, field‐free differential pumping section and passed into a 60° magnetic deflection mass spectrometer. The number of ion‐molecule collisions in the drift tube may be varied over a wide range by changing the source position and/or the gas pressure. Information concerning the nature and probability of the reactions occurring is revealed by the resulting changes in the ionic mass spectrum.