A series of silica-embedded nanocrystalline titania powders with variable Si/Ti ratios have been prepared by ultrasound-assisted hydrolysis. The prepared samples were characterized by thermal analysis (DTA-TG), X-ray diffraction (XRD), FT-IR spectroscopy, nitrogen adsorption-desorption, scanning electron microscopy (SEM), X-ray energy dispersive spectrometer (EDS), and UV-Vis spectroscopy. Photocatalytic activity was checked by the photocatalytic oxidation of formaldehyde in air. Correlations between the microstructure and the photocatalytic activity of the prepared powders were observed. The results showed that improved photocatalytic activity relative to titania-only materials can be made by fabricating silica/titania composite materials through ultrasound- assisted hydrolysis processing, the highest photocatalytic activity was obtained when silica content was 10 wt.% and three times higher than that of Degussa P25. The improved photocatalytic activity was achieved by quantization of particle size, increase of surface area, and hydroxyl content through embedding silica into anatase Titania matrix. (C) 2004 Elsevier B.V. All rights reserved.