The microwave attenuation of air in Mach range 8–12 is investigated at initial pressures of 5 mm Hg. It is shown (a) that the experimental peak attenuation is larger than given by conventional theory; (b) that the time dependence of the attenuation curve gives important information regarding the build‐up of the ionization as a function of time, the contact surface and the zone behind it.It is found that the microwave attenuation rises approximately linearly as a function of time to its peak value. The testing time is shorter than that predicted from ideal shock theory but sufficient to establish equilibrium. The electron density changes abruptly upon arrival of the driver gas. However, an appreciable electron density exists over a large region behind the contact surface indicating strong interface mixing. It is also shown that the nature of the surface of the shock tube influences the peak attenuation and the time dependence of this attenuation.