The electrical properties of Mg‐implanted GaAs have been studied in detail using the van der Pauw technique. Mg ions were implanted in Cr‐doped semi‐insulating GaAs at 120 keV to doses of 1×1013–3×1015/cm2at room temperature. The implanted samples were annealed with rf‐plasma‐deposited Si3N4encapsulants at temperatures ranging from 600 to 850 °C.p‐type layers have been produced for all dose levels. The maximum electrical activation occurred at an annealing temperature of 750 °C for all except the lowest dose of 1×1013/cm2. An electrical‐activation efficiency as high as 85% was obtained for the sample implanted to a dose of 3×1013/cm2and annealed at 750 °C. The dopant profiles were very sensitive to annealing temperature and ion dose, broadening significantly at or above 750 °C. Double peaks in the depth profiles have been observed for doses of ?1×1014/cm2, and the maximum carrier concentration obtained was as high as 2×1019/cm3.