The annealing of n-type thin GaN films grown by metal-organic chemical vapour deposition in vacuum has been studied by beam-based positron annihilation spectroscopy. The results are consistent with a model in which Ga vacancies (V-Ga) exist alongside dislocations and are stable up to 900degreesC. It is suggested that dislocations are shallow positron traps. Upon annealing at,less than or equal to500degreesC the decrease of dislocation density increases the effective positron diffusion length (L+eff) and the probability of trapping at V-Ga. While L+eff continues to change, the trapping of positrons at V-Ga is saturated upon annealing above 500degreesC. The formation of N vacancies near the surface at high temperatures is considered to introduce a potential that retards positron back-diffusion. At 900degreesC dissociation of GaN at a rate of similar to5 nm s(-1) is observed. Oxygen clusters, stable up to 900degreesC, appear to exist near the interface between the GaN film and the sapphire substrate.