It is demonstrated that microwave cutoff can be used to measure absolute electron densities for CHF3, Ar, and O2plasmas in a batch‐mode planar reactor excited at 13.56 MHz. The densities obtained are between 1015and 1016m−3for all gases and a large set of conditions (pressure, rf power, electrode spacing). A comparison with selective etching of SiO2over Si in the same reactor shows a qualitative agreement between the substrate ion flux calculated from the electron density and the etch rate of silicon oxide; the apparent sputter yield ranges from 0.4 to 1.4 molecules per ion.