The reactive ion etching of (100) GaAs in pure CCl2F2and CF4discharges, as well as in mixtures of Ar and CCl4, CCl2F2, or CF4, has been investigated. Anisotropic etching with removal ratesRof up to 800 nm/min have been obtained in reactive discharges operated at a pressure of 5.3 Pa (40 mTorr) and a target voltage of −3 kV. The physical sputtering rate in pure Ar discharges operated under the same conditions was only 40 nm/min. A combination ofinsituoptical emission and absorption spectroscopies have been used to show that in both pure and dilute (up to 90 mole % Ar) halocarbon discharges, physical sputtering of atomic Ga and As is not a primary etching mechanism for GaAs, although ion bombardment does play a critical role in the overall process. Transient glow discharge optical spectroscopy measurements demonstrated that whileRincreased with increasing Cl/F ratios in the etch gas, the steady state carbon concentration at the GaAs surface also increased indicating that carbon accumulation is not the rate limiting step to etching. Rather, the rate limitation is provided by the desorption kinetics of gallium halides which we believe are ejected primarily (except in pure CCl4) through ion‐assisted processes as the reduced radicals GaFxand/or GaClx(x=1 or 2). A phenomenological model is proposed to provide a qualitative description of the etching behavior of GaAs in mixed halocarbon/inert gas discharges.