Aluminium oxide thin films were prepared by the ALE process using AICI3 and A1(OR)3 (R = Et,Pr) as aluminium source and H2O, O2 or various aliphatic alcohols as oxygen source. The process was also operated in the CVD mode and the deposition rates and mechanisms were compared. The films were characterized by Auger and XRF spectrometries for trace element impurities while the chloride residues were quantitatively determined by chemical analysis. FTIR was used to study the residual OH groups. The measured electrical constants and environmental stability indicate that the films are suitable for practical applications as insulating and protective layers.