Gallium oxide thin films were deposited on alumina and TiO2 substrates by metal organic chemical vapour deposition using gallium tris-hexafluoroacetylacetonate (Ga(hfac)(3)) as precursor in the presence of oxygen. The depositions were carried out at 470 degrees C and 2.6 kPa leading to a growth rate of 0.7 mu m h(-1). As-grown films appeared black, smooth and well adherent to the substrates. X-Ray photoelectron spectroscopy (XPS) analysis revealed stoichiometric Ga2O3 with a low carbon content (less than 5%) and almost undetectable fluorine. As-deposited Ga2O3 films were X-ray amorphous, but the onset of a crystallization process was evident after annealing in dry air at 700 degrees C. Moreover, after thermal treatment, the films became carbon free and transparent in the visible range. The effect of annealing at different temperatures on the film structure was investigated by X-ray diffraction. A phase modification from amorphous to polycrystalline Ga2O3 was observed after thermal treatment at temperatures from 600 to 1000 degrees C. As shown by secondary ion mass spectrometry (SIMS) studies, Al diffusion into Ga2O3 films was detected after annealing at 1000 degrees C when pure Al2O3 was used as substrate; this diffusion was completely inhibited using a TiO2 film as buffer layer with a thickness of at least 4 mu m.