The measurement of ionization rates, electron density, and ionization time in thermally ionized gases, by means of a microwave technique, is discussed. The technique used in this work utilizes a so‐called ``end‐wall'' microwave resonant cavity. It is essentially a microwave cavity operated in the TE011mode with a plasma forming one end wall. Since the resonant frequency of such a cavity is a function of the axial length of the cavity, a variable density plasma with its corresponding variable conductivity is utilized to tune the cavity to different resonant frequencies. It is shown that by applying two frequencies to the cavity separated from each other by a known &Dgr;f, which is small compared to the resonant frequency of the closed cavity, a simple method is arrived at to measure electron density, ionization rate, and ionization time.