The rf impedance of a dry etching system excited at 13.56 MHz is calculated from the settings of the matching network between the generator and the reactor. It is demonstrated that fundamental plasma parameters, which are useful in the evaluation of dry etching techniques, can be derived from these measurements. For an oxygen discharge at 27 Pa and 350 W m−2the following parameter values are obtained: maximum ion bombardment energy 76 V, electron density in the glow 5×1014m−3, ion flux density on the substrates 0.08 A m−2.