The behaviour of As‐related defects in SI (semi‐insulating) GaAs are studied from the viewpoint of the origination of As‐related precipitates, generation and evolution of EL2 during postgrowth and the possible defect interactions involved EL2 variation under As overpressure annealing. The precipitates were identified as elemental As and As‐rich GaAs polycrystalline grains and their features depend on the growth process. A defect interaction model has been proposed on the generation/annihilation of EL2 assigned as AsGa VAs VGa complex respect to non‐stoichiometry such as Asi VGa and VAs.