In-grown group III (cation) vacancies (V-Ga, V-Al, V-In) in GaN, AlN and InN tend to be complexed with donor-type defects on the N sublattice, such as O-N or the N vacancy (V-N). The cation vacancies and their complexes are generally deep acceptors, and hence they compensate for the n-type conductivity and add to the scattering centers limiting the carrier mobility in these materials. This work presents results obtained with positron annihilation spectroscopy in a variety of GaN, AlN, InN samples from different sources. The vacancy-donor complexes are different in these three materials, and their importance in determining the opto-electronic properties of the material varies as well. The formation of these defects is discussed in the light of the differences in the growth methods. (C) 2011 Elsevier B.V. All rights reserved.