Thin films of tantalum oxynitride were prepared by reactive magnetron sputtering using a Ta target and N2 and O2 as reactive gases. The nitrogen flow was kept constant while the oxygen flow was pulsed periodically. The film composition evolves progressively from TaO0.25N1.51 to TaO2.42N0.25 while increasing the oxygen pulse duty cycle without any abrupt change in the elemental content. The optical transmission spectra of the films deposited on glass show a "blue shift" of the absorption edge with increasing oxygen content. X-ray diffraction (XRD) patterns of all films exhibit broad peaks typical for nanocrystalline materials. Cross-section film morphology is rather featureless and surface topography is smooth exhibiting very small grains, in agreement with the results obtained by XRD. The optical properties of the films are very sensitive to their chemical composition. All films exhibit semiconducting behaviour with an optical band gap changing from 1.85 to 4.0 eV with increasing oxygen content. In order to evaluate the potential of the tantalum oxynitride films for microelectronic applications some Ta-O-N films were integrated in a MOS structure. The results of the capacitance-voltage measurements of the system Al//Ta-O-N//p-Si are discussed with respect to the chemical composition of the Ta-O-N films. © 2006 Elsevier B.V. All rights reserved.