TiO2 catalysts were prepared by using a modified sol-gel technique and were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and N-2 physi-adsorption methods. The effects of different synthesis parameters on the size and morphology of the TiO2 particles were investigated. A method for obtaining TiO2 with different primary and secondary particle sizes was developed. By changing the water concentration during the hydrolysis of titanium isopropoxide, amorphous titania gel spheres of well-defined morphology and particle size were prepared. These gel spheres define the secondary particle or aggregate size. Following controlled thermal and hydrothermal treatments the primary TiO2 particles (anatase) were formed from the gel spheres without alteration of the aggregate size and shape, Anatase TiO2 catalysts with crystal size of 2.3-30 nm and aggregate size of 100-900 nm were prepared for the gas-phase photo-oxidation of trichloroethylene (TCE), The catalyst performance for TCE degradation exhibited strong dependence on both primary and secondary particle sizes of TiO2, (C) 2000 Academic Press.