High‐temperature annealing of semi‐insulating GaAs has been studied. Thermal conversion induced by annealing at 1060–1200 °C can be explained by the causes of both the reduction of EL2 concentration and the generation of deep acceptors during high‐temperature annealing. Both of them can be rationalized by the supposition that the antisite defect, AsGa, breaks into AsIandVGa, and the latter is a deep acceptor.