Positron lifetime and Doppler broadening spectroscopy were applied for a study of defect properties of semi-insulating GaAs after diffusion of copper. A 30 nm layer of Cu was deposited by evaporation to the undoped GaAs samples. The diffusion of Cu was performed during an annealing step at 1100 ° C at different arsenic vapor pressures. The samples were quenched into room temperature water. The initial semi-insulating SI undoped GaAs sample shows no positron traps. After annealing, a vacancy-type complex and a shallow positron trap were observed to be ef?cient positron traps. Due to the Cu contamination during the annealing process, the shallow trap is believed to be the CuGa double acceptor. The nature of the vacancy-type defects could not be determined unambiguously. The concentration of these vacancies shows inverse relationship to the As vapor pressure that refers to the arsenic vacancy as a part of this complex. Moreover, Doppler-coincidence spectroscopy shows clearly that Cu atoms are not bound in the direct vicinity to the observed vacancies.