Rapid thermal annealing has been used to study the electrical activation mechanisms for magnesium and selenium implants in GaAs. By analysing the changes in electrical activity as a function of annealing time and temperature, a model has been developed which accurately predicts the electrical properties following the post-implant annealing stage. The model has been used to study the activation of other ions, particularly zinc, beryllium, tin and sulphur, the results of which will be compared with those of magnesium and selenium. The results suggest that the mechanism for electrical activation is dominated by the diffusion of gallium, arsenic or vacancies. The paper will present the model and discuss the activation mechanisms of the ions.