Titania nanoparticles were prepared by controlled hydrolysis of titanium tetraisopropoxide (TTIP) in water-in-carbon dioxide(W/C) microemulsion using ammonium carboxylate perfluoro-polyether(PFPE-NH4) as a surfactant. In addition, TiO2/SiO2 nanoparticles were also prepared from TEOS as a silicon source and TTIP as a titanium source. These particles were characterizd by TGA-DTA, XRD, FT-IR and TEM and the photocatalytic decomposition of p-nitrophenol was also investigated using batch reactor in the presence of UV light. It was found that the residual hydroxyl group and the organic compounds were completely removed by calcination at 350degreesC and the amorphous phase transformed to anatase structure above 450degreesC, though the particles dried at 105degreesC, without calcination were amorphous. The crystallinity and crystallite size of nanoparticles produced in water-in-carbon dioxide increased with an increase of W. ratio. The crystallite size of TiO2/SiO2 particles slightly decreased compared to pure TiO2. From FT-IR analysis, the band for Ti-O-Si vibration was observed and the band intensity for Si-O-Si vibration increased with an increase of the silica content. In the photocatalytic decomposition of p-nitrophenol, the photocatalytic activity was mainly determined by the crystallite size of titania and the reaction rate increased with a decrease of crystallite size. In addition, TiO2/SiO2 particles showed higher photocatalytic activity than pure TiO2 on the photocatalytic decomposition of p-nitrophenol. (C) 2004 Elsevier B.V. All rights reserved.