Reactive ion etching of GaAs and AlGaAs has been performed in a BCl3–Ar discharge. Etching properties have been studied as functions of BCl3percentage (0%–100%), total pressure (2.5–30.0 mTorr), and power density (0.06–0.22 W/cm2). At low pressures (2.5–12.5 mTorr) and intermediate BCl3percentages (25%–75%), profiles exhibiting a high degree of anisotropy are achieved. Under anisotropic conditions, etch rates are about 0.05–0.1 μm/min (at power density of 0.22 W/cm2), somewhat lower than for other chlorine‐containing gases. Conditions for etching GaAs and AlGaAs at equal rates have been determined. There is a small lag time between ignition of the glow discharge and the start of etching. The relative concentration of Cl atoms in the plasma, as measured by optical emission actinometry, correlates well with the etch rate for various operating parameters.