Precursor waves have been studied in an electromagnetic shock tube of the T configuration. The precursor velocity, electron density, and current content were determined over a pressure range of 1 to 1000&mgr; Hg, for electrode discharge voltages of 3 to 17 kV in hydrogen and argon. A longitudinal nanosecond microwave technique was used in conjunction with a semi‐infinite microwave cavity measurement to measure the precursor velocity and attendant detailed electron density structure. The current associated with the precursor has been deduced from transverse magnetic field measurements made with an external pickup coil. The precursor exhibits a definite wave structure with front thicknesses of the order of 2 to 5 cm.