The growth of ZnO films deposited by Closed Space Vapor Transport (CSVT) on sapphire substrates has been investigated. Deposition on R oriented sapphire substrates gives rise to a-(11-20) oriented ZnO films. Under optimised conditions, flat surfaces can be achieved and rocking curves with full half width below 500 arcsec are observed. The electrical properties of the films were studied. Hall measurements reveal that the measured n-carrier concentration decreases linearly upon the thickness of the sample. This is interpreted as interface conduction probably related to diffusion of aluminium from the substrate. On thinnest films, the n-carrier concentration can be dramatically decreased with thermal annealing under oxygen. Furthermore, the effect of this annealing under oxygen is found to be completely reversible after a further thermal annealing under oxygen free atmosphere.