Gallium arsenide layers, Si-doped at concentrations of 2×1019, 1×1019, and 5×1018 cm−3, grown on SI substrates were implanted using multiple-energy regimes, with O+, He+, and H+, respectively, to produce resistive structures. Sample resistivity was measured following annealing in the temperature range 400–800 °C. Maximum resistivity values were achieved after annealing at 600 °C for the O+ and He+ and 500 °C for the H+ implanted sample. Equivalently implanted and annealed semi-insulating GaAs samples were analyzed using positron annihilation spectroscopy in the gamma-ray Doppler-broadening mode, a technique which is predominantly sensitive to negatively charged, or neutral, vacancy-type defects. The annealing behavior of the resistivity is in good agreement with previous reports. Vacancy defects to which the positron is sensitive are found to be removed from all semi-insulating samples at a temperature which is 100 °C below that at which maximum resistivity is achieved. Therefore, such vacancy types can be eliminated as the defect responsible for optimum electrical isolation of GaAs following implantation, and the source of vacancies necessary to annihilate such defects. © 2000 American Institute of Physics.