The thermal stability of optical functional coatings is of prime importance for high-temperature applications. TiO-N-x(y) films were deposited by reactive dc magnetron sputtering at room temperature. The influence of the annealing temperature on the structural, mechanical and optical properties of amorphous TiOxNy films was investigated. Rutherford Backscattering Spectroscopy (RBS) measurements showed no variation in the stoichiometry upon annealing for samples prepared in pure oxygen, while the samples prepared in an oxygen/nitrogen mixture were oxidized to TiO2 above 300 degreesC. X-ray diffraction showed that oxidation and crystallization are decoupled. The crystallization temperature is shifted to higher temperatures with increasing amount of nitrogen. Samples that were prepared in pure oxygen or containing only a small amount of nitrogen, crystallize to TiO2 with anatase phase around 300 degreesC, while samples containing a relatively high amount of nitrogen crystallize above 450 degreesC to TiO2 with rutile phase. Crystallization is accompanied by a mechanical stress build up and film densification as revealed by wafer curvature and X-ray reflectivity measurements, respectively. For all samples, the surface roughness increases with increasing annealing temperature. For the films containing a relatively high amount of nitrogen the transparency and the band gap increase, and the refractive index decreases upon annealing. (C) 2004 Elsevier B.V. All rights reserved.