Chlorine was used to carry out the reactive ion etching (RIE) of GaAs. At 5 mTorr pressure and 270 V bias, it was found to etch at a rate>1 μ/min, and produced features having vertical sidewalls and a clean substrate surface. The mechanism of chlorine RIE of GaAs was studied by examining etch rates and profiles obtained using gases of different mixtures of chlorine and argon. The results are consonant with proposed mechanisms of GaAs etching by chlorine in both the plasma and reactive ion beam etching regimes.