The phase transformation of anodized Ti film has been studied. Although X-ray diffraction detected only the amorphous TiO(2) phase, transmission electron microscopy analysis showed that TiO nanocrystallites less than 10 nm in size were also present, which was further supported by x-ray photoelectron spectroscopy analysis. Anatase was found to appear gradually by annealing the as-anodized specimen in air at 500-550 degrees C, which was accompanied by a simultaneous disappearance of TiO nanocrystallites. In contrast, only rutile is formed by annealing the Ti film at the same temperatures. The results indicate that TiO can induce the formation of anatase, which is explained by the close similarity between their structures. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3615999] All rights reserved.