The average electron densities in CH4/H2/Ar and CCl2F2/O2electron cyclotron resonance discharges typical of those used for etching InP, GaAs, and related materials have been measured approximately 4 cm downstream from the multipolar microwave source as a function of microwave power (50–300 W), additional radio‐frequency power (10–50 W), pressure (1–20 mTorr), flow rate (30–90 standard cubic centimeters per minute) and gas composition. At 1 mTorr pressure and 10 W rf, the electron densities (and semiconductor etch rates) increase rapidly with microwave power, from 1.3×1011cm−3for 5CH4/17H2/8Ar and 6×1010cm−3for 28CCl2F2/2O2discharges at 50‐W microwave power, to 9×1011cm−3and 3×1011cm−3, respectively at 300‐W microwave power. At the highest microwave power levels (≥200 W) the InP and GaAs etched surface morphologies are rough due to preferential removal of one of the lattice constituents from each material. The electron densities in both types of discharge show moderate increases with increasing rf power level, pressure or higher Ar or O concentrations.