TiO2-photocatalyzed reaction was conducted in pure benzene in which TiO2 particles were suspended. Benzene was oxidatively decomposed into CO2, as most of organic compounds. However, the rate of CO2 evolution from benzene was gradually lowered as the reaction continued, which was in contrast to the cases of other non-aromatic organic compounds, such as alcohols or acids. To clarify the reason for the decrease in activity of TiO2, intermediates produced in the solution phase and on the surface of TiO2 were analyzed using chemical and physical techniques. Several kinds of intermediates were identified: phenolic compounds, aldehydes, carboxylic acids, and polymeric substance. Of these products, catechol and polymeric substance, which were observed only on the surface of TiO2 powder, were found to be responsible for the lowering of the photocatalytic activity of TiO2. (C) 2009 Elsevier B.V. All rights reserved.