Nanocrystalline titanium oxinitride (TiO2-2xNx) thin films (0 <= x <= 1) were prepared by reactive dc magnetron sputtering from a titanium target in an argon-oxygen-nitrogen atmosphere. By increasing the reactive gas component nitrogen the phases changed from TiO2 to TiO2-2xNx and finally to TiN. The reactively sputtered films were characterized by elastic recoil detection analysis, Raman spectroscopy, and resistivity measurements. The crystallographic structures of TiO2 were maintained up to a nitrogen concentration of about 20 at. %; however, the crystallite size decreased significantly and a transition from the low-temperature anatase to the high-temperature rutile phase of TiO2 occurred. Optical transmission measurements revealed that the indirect energy band gap can be reduced from 3.2 to 2.6 eV for nitrogen concentrations up to 20 at. %. The film properties for photocatalytic water oxidation were investigated by differential electrochemical mass spectroscopy. While the photoactivity in the visible increases, the overall photoelectrochemical activity appears to be deteriorated significantly by nitrogen doping, most probably due to the formation of defect states near to the valence band of TiO2. (c) 2006 American Vacuum Society.