The hydroxylation process is the primary, and even the rate-determining step of the photocatalytic degradation of aromatic compounds. To make clear the hydroxylation pathway of aromatics, the TiO2 photocatalytic hydroxylation of several model substrates, such as benzoic acid, benzene, nitrobenzene, and benzonitrile, has been studied by an oxygen-isotope-labeling method, which can definitively assign the origin of the O atoms (from oxidant O2 or solvent H2O) in the hydroxyl groups of the hydroxylated products. It is found that the oxygen source of the hydroxylated products depends markedly on the reaction conditions. The percentage of the products with O2-derived hydroxyl O atoms increases with the irradiation time, while it decreases with the increase of substrate concentration. More intriguingly, when photogenerated valence-band holes (hvb+) are removed, nearly all the O atoms (>97 %) in the hydroxyl groups of the hydroxylated products of benzoic acid come from O2, whereas the scavenging of conduction-band electrons (ecb−) makes almost all the hydroxyl O atoms (>95 %) originate from solvent H2O. In the photocatalytic oxidation system with benzoic acid and benzene coexisting in the same dispersion, the percentage of O2-derived hydroxyl O atoms in the hydroxylated products of strongly adsorbed benzoic acid (ca. 30 %) is much less than in that of weakly adsorbed benzene (phenol) (>60 %). Such dependences provide unique clues to uncover the photocatalytic hydroxylation pathway. Our experiments show that the main O2-incorporation pathway involves the reduction of O2 by ecb− and the subsequent formation of free .OH via H2O2, which was usually overlooked in the past photocatalytic studies. Moreover, in the hydroxylation initiated by hvb+, unlike the conventional mechanism, the O atom in O2 cannot incorporate into the product through the direct coupling between molecular O2 and the substrate-based radicals.