The hydrophilic and photocatalytic properties of the SiO2/TiO2 double layers composed of a 20-nm-thick porous SiO2 layer on the 200-nm-thick columnar anatase TiO2 layer were studied. The hydrophilicity of the double layers was strictly determined by the relative coverage of organic contaminants. The intrinsic hydrophilicity of 0 degrees of SiO2 in terms of the water contact angle was restored by the photocatalytic decomposition of organic contaminants Under the UV light irradiation. Electron spin resonance measurements revealed the generation of OH radicals under the UV light irradiation onto the SiO2/TiO2 double layers. Photoconductivity measurements showed that the Current decay in O-2 gas atmosphere was remarkably fast in comparison with that in H2O vapor. These observations Support our view that the generation of Oil radicals effective for decomposing organic contaminants on the surface begins with the reaction between O-2 Molecules and the photoexcited electrons. We propose together with other experimental facts herein that OH radicals would be generated via O-. (2) over bar and H2O2 in the double layer system. (c) 2005 Elsevier B.V. All rights reserved.