Abstract Experimental results obtained over the last three decades on photoinduced oxygen isotopic exchange (POIE) of TiO 2 oxygen atoms with those of adsorbed water molecules and gaseous O 2 are analyzed in the light of recent information from the literature on the interaction of water and O 2 species with the TiO 2 surface (obtained by application of surface spectroscopy techniques in combination with high‐resolution scanning tunnelling microscopy). The analysis emphasizes the singular role that bridging oxygen ions and bridging oxygen vacancies play in TiO 2 surface chemistry and interfacial electron transfer at the gas phase–TiO 2 interface in the absence and presence of water. The observed competition between POIE and the photo‐oxidation (PO) of organic compounds is analyzed in terms of the recently developed direct‐indirect (D‐I) kinetic model for heterogeneous photocatalysis (D. Monllor‐Satoca et al., Catal. Today , 2007 , 129 , 247, and references therein).