Nanostructured titanium dioxide (TiO2) photocatalytic coating was deposited through flame spraying with liquid feedstocks. The effect of annealing treatment on the microstructure of the TiO2 coatings was investigated. The microstructure of the coatings was characterized by x-ray diffraction, thermal analysis and transmission electron microscopy. The photocatalytic performance was characterized by photodegradation of gaseous acetaldehyde. The results showed that the as-sprayed nanostructured titania deposited by flame spraying with liquid feedstocks is mainly composed of anatase phase. X-ray diffraction analysis showed that annealing at a temperature higher than 600 degreesC results in transformation of the anatase phase to rutile phase. Transmission electron microscopy examination revealed that the particles in the deposit remain in their original nano-size when annealing temperature is lower than 800 degreesC. Thermal analysis suggested that the transformation from anatase to rutile may start at a temperature of around 400 degreesC. On the other hand, the results evidently indicated that photocatalytic performance is not directly related to the anatase content in the coating. The onset of the phase transformation dramatically degrades the photocatalytic activity. Those facts suggest that the transformation of the anatase phase to rutile phase begins at the surface of the particles which may result in the formation of a surface shell of the rutile phase TiO2. (C) 2004 American Vacuum Society.