Electron paramagnetic resonance spectra of fast neutron irradiated semi‐insulating GaAs, recorded at 9 GHz and 4.2 K, have been studied as a function of isochronal thermal anneals. Their decomposition into quadruplet and singlet allows one to determine the main annealing temperatures of the corresponding defects, previously identified as As4+Gaand V2−Ga, which occur respectively at 400 and 600 °C. Comparison with the behavior of implantation damage shows that electrical activation of Be+implants is correlated with the annealing of the main defects on the cation sublattice. Finally, the linewidth variation of the quadruplet during its decay indicates a concomitant change of the local environment of the As4+Gacenter.