The mass spectrometer has frequently been employed to study chemical kinetics and ionization processes. The difficulty encountered is the sampling process, i.e., the transfer of a sample of gaseous or ionized species from a high‐temperature and high‐pressure reactive region, into the operating pressure of a mass spectrometer,10−5mm Hg, with the fewest reactions and changes of composition. The sampling process in the shock tube is further complicated by the nonsteady, nonuniform, cool, and dense end‐wall thermal layer. A detailed study of ions (or gaseous species) diffusing through the thermal layer and expanding through an orifice to a free‐molecular jet, and the reactions which occur during the process, is carried out using a simplified model. The agreement between the simplified model results and the experimental results promises the possibility of obtaining quantitative measurement by this sampling process.