Experiments in an electric shock tube operating at a few Torr have shown compression waves moving ahead of the luminous discharge plasma for Mach numbers below 12 to 15. Failure to observe this separation at higher velocities is attributed to mixing caused by a Taylor unstable density distribution and the deceleration of the flow. The unstable distribution is caused by the density rise accompanying ionization behind the shock front. A model based on these assumptions, which correctly predicts separation conditions in argon, is presented. The structure of the shock waves (pressure, flow velocity, and temperature) is approximately described by blast wave theory. The deviations are primarily due to real gas effects.