The application of microwave interaction with gaseous plasmas to the study of electrically driven shock waves in nonionized gases and gaseous plasmas is discussed. The microwave measurements are supplemented by observation of the visible light emitted from the shocked gas and by voltage induced signals on small metallic probes placed in the path of the propagating shock waves. The investigated shock waves were in the Mach 4 to Mach 14 velocity range in neon and argon gases at pressures from 0.1 to 5 mm Hg. It is demonstrated, in agreement with other workers, that the background gas through which the shock waves subsequently passes is ionized to a nonnegligible degree at the instant of the electric discharge by photons of adequate energy from the discharge chamber. It is also shown that at least in the particular shock tube used in this investigation, this preionization is consistent with ionization by soft x rays from the electron bombarded anode in the discharge chamber. The effect of the free electron constituent of the ionized background gas on the properties of the shock wave as well as its direct effect on some commonly used shock wave investigation techniques is discussed.