The paper reports on thermal stability of alumina thin films containing gamma-Al2O3 phase and its conversion to a thermodynamically stable alpha-Al2O3 phase during a post-deposition equilibrium thermal annealing. The films were prepared by reactive magnetron sputtering and subsequently post-deposition annealing was carried out in air at temperatures ranging from 700 degrees C to 1150 degrees C and annealing times up to 5 h using a thermogravimetric system. The evolution of the structure was investigated by means of X-ray diffraction after cooling down of the films. It was found that (1) the nanocrystalline gamma-Al2O3 phase in the films is thermally stable up to 1000 degrees C even after 5 h of annealing, (2) the nanocrystalline theta-Al2O3 phase was observed in a narrow time and temperature region at >= 1050 degrees C, and (3) annealing at 1100 degrees C for 2 h resulted in a dominance of the alpha-Al2O3 phase only in the films with a sufficient thickness. (C) 2010 Elsevier B.V. All rights reserved.