The reactive ion etching of (100) GaAs in CCl2F2and mixed CCl2F2+Ar discharges has been investigated. Anisotropic etching with removal ratesRof 0.8 mm/min have been obtained in pure CCl2F2discharges operated at 5.3 Pa (40 mTorr) and −3 kV, whereas the physical sputtering rate in pure Ar discharges under these conditions was only 0.04 mm/min. A combination of optical emission and absorption spectroscopies have been used to show that in CCl2F2and even in dilute 10% CCl2F2+90% Ar discharges operated under the same conditions, physical sputtering is not a primary etching mechanism for GaAs, although is is important for minimizing residual carbon accumulation. Rather, etching is believed to occur by the formation and desorption of Ga and As halides with the rate limiting step being the removal of GaF3. The kinetics of this process are aided by ion‐bombardment‐enhanced diffusion and stimulated desorption.