The generation of OH radicals on UV-illuminated TiO2 surface is mainly responsible for the photocatalyric oxidation of pollutants in various contaminated environmental media. Although the reactivity of OH radicals is largely limited within the surface region, the possibility of OH desorption and diffusion into the reaction medium has been often raised. This study provides several examples for the presence of diffusing OH radicals in aqueous solution and polymer matrix containing TiO2 particles. The photocatalytic degradation rates of (CH3)(4)N+ in TiO2 suspension were comparable between acidic and alkaline conditions, which could not be explained by a simple electrostatic surface charge model. From the present mechanistic study, it is suggested that the photocatalytic oxidation of (CH3)(4)N+ at acidic pH mainly proceeds through free OH radicals in the solution bulk, not on the surface of TiO2. The diffusing OH radicals also played the role of main oxidants in the solid phase. The photolysis of TiO2-embedded PVC composite films generated cavities around the imbedded TiO2 particles and the development of cavity diameter continued even after the direct contact between the PVC and TiO2 was prohibited. This implied that active oxygen species that were photogenerated on TiO2 surface desorbed and diffused across a few micrometers to react with the polymer matrix.