Electronic properties of Si and Mg implants in undoped semi‐insulating GaAs are studied. The activation of the implants is achieved by rapid thermal annealing. The effects of implantation dose and anneal temperature on the measured electrical activity are investigated. In spite of similar depth distributions and implantation damage characteristics, a marked difference between the activations of the Si and the Mg ions is observed for the dose range considered (3×1012–1×1014cm−2). Lattice strain measurements performed by x‐ray rocking curves indicate that the residual implantation damage after annealing is not largely responsible for this difference. The difference is mostly electronic in character, as also suggested by photoluminescence measurements. At high annealing temperatures, changes in the compensating properties of undoped semi‐insulating GaAs are suspected, and are found to play an important role in the activation of implanted ions, affecting then‐ andp‐type dopants conversely.