The characteristics of a CCl2F2:He reactive ion etch are reported for use in selectively etching GaAs/AlGaAs heterostructures. The etch is performed at a pressure of 150 mTorr, with rf power of 0.5 W/cm2. Both He and CCl2F2flow rates are controlled to 10 sccm, with measured dc self‐bias of typically 125 V. The etch rate for GaAs is 1.8 μm/min, while the AlGaAs etch rate depends on the Al mole fractionxwith measured values of 150 Å/min atx=0.28 and 45 Å/min atx=0.5. The selectivity is 120 atx=0.28 and 420 atx=0.5. The etch shows an anisotropy between the 〈011〉 and the 〈011̄〉 directions. Measurements of the plasma impedance are reported as a function of pressure, power, and gas flow rate. From these measurements, the dc bias can be computed and is shown to give good agreement with the measured values. The plasma conditions are sensitive to the previous history of the chamber, but by preconditioning the chamber, the etch is readily reproduced.