Preionization electron densities have been studied in an x‐ray preionized XeCl laser as a function of the gas composition. Also the influence of emission of photoelectrons from the electrode surfaces on the initial electron density has been determined. For low Xe partial pressures, an excited neon bottleneck in the Penning ionization process of Xe is found. For higher Xe pressures, the preionization density increases linearly with the Xe concentration. A simple model gives a satisfactory agreement with the measurements over more than two decades of Xe partial pressure. We also determine the density of Ne* following x‐ray irradiation. Measurements in mixtures containing HCl show that due to electron attachment no initial value of the electron density can be assigned. The method used here will give a lower estimate for the total charge density in these mixtures.