TiO2-SiO2 mixed oxides were prepared by sol-gel processes with one-stage (mix up fully hydrolyzed titania- and silica-sol), two-stage (with pre-hydrolysis) and modified two-stage synthesis routes. The photoresponse and AC impedance characterization of the derived catalysts are studied and correlated for the first time with the photocatalytic activities in water decomposition under UV illumination. Synergistic effects in terms of photocatalytic activity and electronic properties including band-gap energy, flat band potential and doping density were observed on atomically mixing TiO2 and SiO2 by the two-stage synthesis route. Meanwhile, the decline of photocurrent density were found on TiO2-SiO2 relative to bare TiO2, which could be attributed to low quality crystalline structure of the former compared to that of the latter. The superior photocatalytic performance of TiO2-SiO2 is ascribed to the higher flat band potential, band-gap energy, and doping density than those of bare TiO2. (C) 2003 Elsevier B.V. All rights reserved.