Antenna voltage breakdown arising from the transmission of high‐power microwave radiation may be suppressed on a reentry vehicle by injecting electrophilic chemicals into the flow field. This paper describes numerical procedures for calculating the amount of high‐power microwave radiation that can be transmitted through a plasma chemically seeded with an electron‐attaching substance. The power incident on a reentry plasma slab may be sufficient to induce electron temperature changes, resulting in variations in time of the electron density and effective collision frequency. The plasma properties are determined as a function of time by numerically integrating the two continuity equations for the electron and positive‐ion densities. The processes of electron impact ionization, electron attachment, detachment, and recombination are taken into consideration. The diffusion of the electrons and positive ions in the ambipolar limit is influenced by the presence of negative ions. Runge‐Kutta numerical integration of Maxwell's equations together with the above set of plasma transport equations yield the time‐dependent plane‐wave transmission and reflection coefficients as a function of incident power level for a very wide range of reentry plasma conditions. For one particular set of reentry conditions, graphs are presented of the electron density profile, reflection coefficient, and transmission coefficient as functions of time.