We have applied positron annihilation spectroscopy to study in-grown vacancy defects in bulk aluminium nitride (AlN) crystals grown by physical vapor transport. We interpret the lowest lifetime value of about 155 ps, measured at low temperatures, to represent the annihilations from the free state of the positron in the crystal lattice. The increased lifetime at high temperatures is an indication of positrons annihilating as trapped at vacancy defects, and a second lifetime component could be separated from the lifetime spectra at temperatures above 400 K. The same lifetime component tau(2) = 210 +/-10 ps was found in all AlN samples. This component call be attributed to positrons annihilating at Al vacancies, possibly complexed with impurities such as oxygen, present at concentrations in the 10(17) cm(-3) range in the samples. In addition to Al vacancies, negatively charged non-open Volume defects acting as shallow hydrogenic traps for positrons were detected, with concentrations of about 10(18) cm(-3) or higher. These defects are the dominant negative acceptor defects in these samples. (c) 2008 Elsevier B.V. All rights reserved.