Herein, we studied the role of bridging oxygen (O-br(2-)) in the photocatalytic degradation reaction of gaseous acetaldehyde under anaerobic conditions. (TiO2)-O-16 and the isotopologues (TiO2)-O-18 were treated with acetaldehyde in the dark and upon UV illumination and at the reactor outlet the evolved (CO2)-O-16 and (COO)-O-18-O-16 were monitored by mass spectrometry (MS). (TiO2)-O-16 exhibited a constant (COO)-O-18-O-16/(CO2)-O-16 ratio in the dark and upon UV illumination. In the dark the (COO)-O-18-O-16/(CO2)-O-16 ratio obtained with (TiO2)-O-18 was similar to the ratio monitored with (TiO2)-O-16, while upon UV illumination the ratio increased evincing an incorporation of O-br(2-) into the formation of CO2. Accordingly, a following reaction mechanism could be derived: upon UV illumination the generated holes are trapped by the bridging oxygens forming O-br(-), which then react with adsorbed acetaldehyde forming acetate that is decomposed into CO2 and CH4. The remaining oxygen vacancy is replenished by an oxygen atom supplied by adsorbed water. This is, to the best of our knowledge, the first time that the crucial role of O-br(2-) within the photocatalytic process has been described for a gas phase reaction.