Nanosized TiO 2 and SiO 2 /TiO 2 particles were synthesized by hydrolysis of tetrabutyl titanate and tetraethyl orthosilicate in a Triton X-100/n-hexanol/cyclohexane/ammonia reverse microemulsion. The particles were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy. The photocatalytic activity was evaluated by photocatalytic degradation of methyl orange under UV light irradiation. The effects of SiO 2 content, crystal structure, and crystallite size on the photocatalytic activity were investigated. From FT-IR analysis, a new band at 950 cm -1 for Ti-O-Si vibration was observed. The Ti-O-Si bond and amorphous SiO 2 suppressed the phase transformation of titania from anatase to rutile, increased the stability of anatase TiO 2 , and inhibited the crystallite growth. The average size of titania decreased from 38.4 nm in pure TiO 2 to 3.9 nm in SiO 2 /TiO 2 with a SiO 2 /TiO 2 molar ratio of 2/1. In addition, the photocatalytic activity decreased with an increase of rutile content. SiO 2 /TiO 2 particles with a SiO 2 /TiO 2 molar ratio of 1/7 showed the highest photocatalytic activity for the photocatalytic degradation of methyl orange.