We have used a low-energy positron beam to identify and quantify the dominant vacancy defects in InN layers grown on Al2O3 by molecular beam epitaxy. By applying both continuous and pulsed positron beams, we can show that In vacancies are formed during the crystal growth. Their concentration decreases from similar to 5 x 10(18); to below loll cm(-3) with increasing layer thickness (120-800 nm). The In vacancy concentration correlates with the free electron concentration and decreases with increasing electron Hall mobility. These results suggest that In vacancies act as both compensating defects and electron scattering centers in InN films. (C) 2004 Elsevier B.V. All rights reserved.