The electron temperature of a shock tube magnetohydrodynamic plasma was measured by the line‐reversal method. In order to reduce the influence of self‐reversal in the cool boundary layer on the measurement, the use of nonresonance lines of the potassium seeding was first explored theoretically, then confirmed by experiment. Best suited lines were selected by integrating the radiative transfer equation over both the wavelength and the width of the shock tube assuming a boundary‐layer temperature profile. A lower limit to the population density of the upper level of the selected transition(4p‐6s)was found from the rate equation for this level. The electron temperature distribution along the plasma column was measured using Moutet's double beam version of the line‐reversal method. The measurements show good correspondence between the temporal evolution of the electron temperature in the center of the flow and the generator current. The discrepancy between the measured electron temperature and the gas temperature calculated from the shock Mach number is explained by the occurrence of nonequilibrium ionization.