Two series of samples were prepared from titanium tetraisopropoxide (TTIP) through hydrolysis with different aging times, 10 min and 24 h, at room temperature and then annealed at different temperatures. Photocatalytic activities of these samples for methylene blue (MB) in water and for diluted NO gas were examined. For MB decomposition, rate constant k was determined from linear relation between logarithm of relative concentration of MB remained in water and UV irradiation time. For NO, its decomposition fraction after 10 h irradiation was determined. Difference in aging time gave different crystalline state of the precipitates, anatase phase with poor crystallinity after 10 min aging, but amorphous state after 24 h aging. These two precipitates showed different changes in crystallinity and in photoactivity for MB and NO with annealing temperature. Maximum rate constant k for MB decomposition was obtained at around 0.6degrees of full width at half maximum intensity (FWHM) of 101 diffraction line. For NO gas, a maximum decomposition fraction was obtained at FWHM of around 1.6degrees. Different crystallinity of anatase was shown to be required for the decomposition of MB in water and NO gas, high crystallinity of anatase phase for the former but poor crystallinity for the latter. (C) 2004 Elsevier B.V. All rights reserved.