Amorphous a-Al2O3 thin films were prepared by vapour deposition using a special Knudsen cell heated by electron bombardment. The Al/O stoichiometry was controlled in situ by Auger electron spectroscopy, and the structure by transmission electron microscopy. It is shown that the stoichiometric layers can be prepared at a cell temperature of up to 2050 K which corresponds to a deposition rate of 3 nm h(-1) at a cell-substrate distance of 50 mm. At higher temperatures alumina molecules start to dissociate and the layers contain free Al atoms. In this case the stoichiometry of the layers can be improved by evaporation under oxygen atmosphere.