Titanium silicalite (TS-1) having Si/Ti ratio of 26.3 was synthesized and used as a photocatalyst for decomposition of 4-nitrophenol (4-NP) in water. The photocatalytic activity of TS-1 for decomposition of 4-NP was found to be greatly affected by H₂O₂. In the absence of H₂O₂, TS-1 exhibited only small catalytic activity irrespective of pH. In the presence of H₂O₂, on the other hand, TS-1 showed a marked increase in photocatalytic activity. Especially, in acidic conditions, the addition of H₂O₂ in reaction solution resulted in a remarkable enhancement of photocatalytic activity. It seemed that titanium-hydroperoxide species are formed by the interaction of framework titanium in TS-1 with H₂O₂ and ·OH radicals are formed easily from titanium-hydroperoxide species under UV illumination. In this experimental condition, most of the 4-NP molecules and ·OH radicals are likely present in tiny volume of the TS-1 pore. As a result, in the presence of H₂O₂, the photocatalytic decomposition of 4-NP over TS-1 can be enhanced due to the efficient formation of ·OH radicals and the close proximity of ·OH radicals to the reactant molecules.