Positron annihilation is an established technique for investigating vacancy-type defects in semiconductors. When a positron is implanted into solids, it annihilates with an electron and emits two 511 keV gamma quanta. From measurements of Doppler broadening spectra of the annihilation radiation and the positron lifetimes, one can detect vacancy-type defects such as monovacancies and divacancies. We used monoenergetic positron beams to study vacancies in ion-implanted and rare-earth-doped GaN. We identified the defect species and estimated their concentrations from a comparison between the Doppler broadening spectra obtained through the experiments and those calculated using first-principles calculation (projector augmented-wave method). We have thus shown that positron annihilation is a useful tool for studying relationships between vacancy-type defects and properties of group-III nitrides. (C) 2009 Elsevier B.V. All rights reserved.