Measurements of electron density behind a shock wave in air are presented. Operating conditions are such that shock‐wave deceleration, boundary‐layer mass loss, and chemical nonequilibrium are important in the determination of the development of the electron density behind the shock wave. A one‐dimensional unsteady analysis is described. Calculated electron density is compared with experimental values. Agreement is within a factor of 2. The difference is probably attributable to expansion waves embedded in the test gas.