A simple and noninvasive technique is demonstrated for measuring the plasma electrical impedance for a class of reactors commonly used in fabricating integrated circuits. These reactors have an adjustable electrical matching network for optimizing the power transfer into the plasma. A procedure for determining in the matching network the needed values of components, which are normally difficult to measure directly, is also developed and illustrated. This dynamic measurement of plasma impedance, when combined with an electrical model of the plasma, is shown to be useful for monitoring the electron density and the electron‐neutral collision frequency in the process plasma.