Titanium dioxide has been the most popular environmental photocatalyst of which role critically depends on the generation of OH radicals. In particular, the mobile free OH racial (OHf) generation and the subsequent diffusion from the surface are critical in achieving the mineralization of non-adsorbing substrates by extending the reaction zone from the surface to the solution bulk. Here the origin of the crystalline phase-dependent generation of OHf was investigated using tetramethylammonium (TMA) cation as a main probe compound for OHf in a UV/TiO2 photocatalytic system. We found a clear evidence that the mobile free OH radical is generated through a reductive conversion of dissolved O2 on anatase only (O2 → H2O2 → OHf). The surface trapped holes are not involved in OHf formation, but lead to the generation of surface-bound OH radical (OHs) on both anatase and rutile. The generation of OHf is favorable on anatase because more H2O2 are evolved (via dioxygen reduction) and adsorbed on the anatase surface. Rutile showed little sign of OHf formation. The generation of 18O-labelled p-hydroxybenzoic acid on anatase only (not rutile) from benzoic acid oxidation under 18O2-saturated condition provides a solid evidence that the OHf generation mechanism on anatase involves the reductive pathway. Better understanding of OHf production pathway in photocatalysis will provide a new insight leading to an engineering solution for how the production of OHf can be maximized, which is critically important in achieving the efficient photocatalytic oxidation of various pollutants.​