Benzene is oxidized to phenol by TiO2-photocatalytic reaction in liquid benzene. However, this reaction decelerates as the reaction is continued because the TiO2 surface is gradually covered with the polymerized products of phenol. Two methods were found to be useful to suppress the polymerization and improve the production of phenol. They were extraction of phenol produced in benzene into alkaline water and use of HF-treated TiO2 powder as the photocatalyst. By combination of these methods, the amount of phenol produced was almost doubled.