A highly adsorptive allophane-TiO2 nanocomposite photocatalyst was prepared by dispersing nanopartides of the natural clay mineral allophane into a titanium alkoxide solution by the sol-gel method. During the photocatalytic degradation of trichloroethylene using the allophane-TiO2 nanocomposite, emission of the intermediate product, phosgene, was drastically inhibited. Trichloroethylene was transformed into the intermediate products, phosgene and dichloroacetyl chloride, on the TiO2 during the UV irradiation. These compounds are rapidly adsorbed on the allophane. The compounds then gradually degraded after diffusing to the TiO2.(C) 2010 Elsevier B.V. All rights reserved.