The electron density in electron‐beam irradiated Ar/HCl gas mixtures has been measured by time‐resolved infrared interferometry at 9.6 &mgr;m, using a Mach‐Zehnder interferometer. Measurements were performed fore‐beam irradiated gas mixtures with various HCl concentrations between 0.05% and 5.0%, and total gas pressures between 0.5 and 4.0 atm. Thee‐beam pulse was 350 ns long and at an energy of 150 keV. Thee‐beam current density was varied between 25 and 14 mA/cm2. The measured electron‐density pulse had a characteristic shape with a maximum at the first 100 ns, followed by a drop toward the end of thee‐beam pulse. The values of the electron density at the maximum and at the end of thee‐beam pulse varied slowly as a function of HCl concentration. The experimental results were modeled by incorporating a simplified kinetic code into theelendifBoltzmann code, which calculated the electron energy‐distribution function. The code calculations took into account the effect of the large cross sections for vibrational and rotational excitation of HCl molecules by electron impact. The main conclusion of the analysis is that these cross sections reduce the electron average energy below 0.5 eV for HCl concentrations higher than 0.1%. This conclusion and the results of the code calculations are qualitatively consistent with the experimental observations. But no quantitative agreement was obtained between the code predictions and the experimental results. Possible reasons for this disagreement are discussed.