In this work, silica nanospheres were prepared in reverse micelles using compressed CO2 first as a reactant and then as an antisolvent. The transmission electronic micrographs (TEM) show that the silica nanoparticles in the size range of 3-15 nm were obtained, and their size increased with an increase in molar ratio of water to surfactant (w) and the decreased pressure. Furthermore, the TiO2-SiO2 core-shell particles were prepared by the reaction of compressed CO2 with sodium silicate on the surface of TiO2 nanoparticles in the reverse micelles. The morphologies, phase structure, interfacial interaction and the optical properties of the TiO2-SiO2 core-shell particles were characterized by transmission electronic micrographs (TEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and UV-vis spectra, respectively. (c) 2005 Elsevier B.V. All rights reserved.