Nanosized titania sol has been produced by the controlled hydrolysis of titanium tetraisopropoxide (TTIP) in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. The physical properties, such as surface area, crystallite size and crystallinity according to R and WO ratio, have been investigated by TEM, XRD, BET, FT-IR, TGA and DTA. In addition, the photocatalytic decomposition of p-nitrophenol has been studied by using a batch reactor in the presence of UV light in order to compare the photocatalytic activity of prepared nanosized titania. It is shown that the anatase structure appears in the 300-600 degrees C calcination temperature range and the transformation of anatase into rutile starts above 700 degrees C. The crystallite size increases with increasing R and W-0 ratio but W-0 ratio shows a stronger effect on the crystallite size than R ratio. In the photocatalytic decomposition of p-nitrophenol, the photocatalytic activity is mainly determined by the crystallinity of titania. In addition, the titania calcined at 500 degrees C shows the highest activity on the photocatalytic decomposition of p-nitrophenol(k = 5.6 x 10(-3) min(-1)) and the pure anatase structure. (c) 2004 Elsevier B.V. All rights reserved.