Undoped GaAs was enriched with copper and studied by means of positron annihilation after quenching. Vacancies and small vacancy clusters (tau_def < 350 ps) were registered by positron lifetime spectroscopy if copper diffuses out after annealing. Doppler coincidence spectroscopy reveals that the vacancies and the vacancy clusters are decorated with copper atoms. Number and size of these microvoids depend on the initial copper concentration.