We have applied positron annihilation spectroscopy to show that 2 MeV electron irradiation at 300 K creates Ga vacancies in GaN with an introduction rate of 1 cm(-1). The Ga vacancies recover in long-range migration processes at 500-600 K with an estimated migration energy of 1.5(2) eV. Since the native Ga vacancies in as-grown GaN are stable up to 1300-1500 K, we conclude that they are complexes with oxygen impurities. The estimated binding energy of 2.2(4) eV of such complexes is in good agreement with the results of theoretical calculations. (C) 2001 Elsevier Science B.V. All rights reserved.