Sheet‐resistivity and Hall‐effect measurements were made on C‐ion‐implanted GaAs after annealings at 600, 700, 800, or 900 °C. A doping efficiency of up to 50% was obtained in the 900 °C‐annealed sample which is much higher than the 2–8% reported previously. The electrical profile of this sample indicated that most of the implanted C became substitutional. The electrical compensation level, on the other hand, remained relatively high (0.3–0.6) throughout the profiling range and is responsible for the low doping efficiency obtained in the C‐implantedp‐type GaAs.