The purpose of this work is to study the evolution of the structure and of the thermal stability of a group of tantalum oxynitride thin films, prepared by magnetron sputtering, under the influence of vacuum annealing, up to a temperature of 800 degrees C. When varying the partial pressure of the reactive gases (PO2+N2), during the deposition process, the films change from a structure with a combination of poorly developed crystallites of the tetragonal beta-Ta and of the face centred cubic (fcc) Ta(O,N) phases, for the films deposited with low PO2+N2, to a quasi-amorphous structure, for the films deposited with highest pressures. For intermediate pressures, the films reveal the presence of the fcc-Ta(O,N) structure. This structure corresponds to O atoms substituting some of the N atoms on the fcc-TaN structure and/or N atoms substituting O atoms of the fcc-gamma-TaO structure. When subjected to the thermal annealing at 700 degrees C or higher, the film produced with lowest partial pressure revealed a remarkable structural change. New diffraction peaks appear and can only be attributed to a sub-stoichiometric hexagonal tantalum nitride structure. The film did not reveal any signs of delamination or cracks after all annealing temperatures. The two films produced with highest partial pressure proved to be the most stable. Structurally, they maintain the amorphous structure after all the annealing treatments and, in addition, no cracks or delamination were detected. (C) 2013 Elsevier B. V. All rights reserved.