Microwave transmission measurements of the electrical properties of shock‐ionized air in the range 8≤Ms≤12 and initial pressures of 1, 2, 5, 10, and 20 Torr are presented. The test arm of the microwave transmission bridge isXband waveguide, which passes through the shock tube and has an affixed nozzle to section out a portion of the shocked gas. This gas is constrained to a rectangular cross section and passes through the waveguide where it interacts with the microwave signal. An 0.5‐in. and a 1.5‐in. nozzle are used, and it is shown that nozzle length has a negligible effect on both the ionization rate and the level. The data are presented in terms of dielectric constant, electron density, collision frequency, conductivity, attenuation, and reflection coefficient. The data are compared with results obtained from other experimental methods and discrepancies are noted. Values ofNeand &ngr; are calculated initially from the Lorentz relation with &ngr; independent of velocity; these data are then recalculated by use of Hochstim's correction factors. Ionization profiles obtained from the microwave measurements at 1 Torr agree belowMs=11 with theoretical values obtained from the rate program of Garr and Marrone. Evaluation of ionization profiles in terms of rate constants is complicated at all pressures by the effects of accelerating shocks.