Be+/O+‐ion implantation is performed in GaAs/AlGaAs heterojunction wafers. Secondary ion mass spectroscopy analysis shows that the oxygen profile is in good agreement with the theoretical profile (LSS) and that the beryllium profile is still deeper than the theoretical one. The electrical properties of the ion‐implanted heterojunctions are studied by means of current‐voltage (I‐V) and capacitance‐voltage (C‐V) measurements and are found to strongly depend on the isochronal annealing temperature. A low‐temperature anneal (≤400 °C) produces a high‐resistivity isolation layer of about 0.4 &mgr;m thick in the heterojunctions. At annealing temperatures above 400 °C, the data indicates a pronounced decrease in the carrier concentration in AlGaAs regions as compared with their original doping, but a good recovery after a high‐temperature anneal (800 °C).