Anatase-type nanocrystalline titania layers were prepared by oxidizing the crystalline titanium nitride and carbon-doped titanium nitride phases prepared on a titanium plate surface. Identification of the crystalline phase was confirmed by the XRD patterns and Raman spectra of these plates. The photocatalytic activity of the plates was evaluated by observing the photocalytic degradation process of acetaldehyde gas during UV irradiation by gas chromatography. A relatively larger amount of the anatase phase was formed on the very thin surface layer by heating the carbon-doped titanium nitride phase at 500 degrees C, and it exhibited a higher photocatalytic activity for the acetaldehyde degradation. The activity was determined not only by the amount of the anatase phase, but also by the crystallite size depending on the surface area and charge transfer efficiency. The photocatalytic activity is suggested to be due to the anatase phase existing on the thin surface layer accessible to the reactants. (C) 2012 Elsevier B.V. All rights reserved.