© 2017 Elsevier B.V. Based on density functional theory we propose R1 mechanism of photostimulated oxygen isotope exchange (POIEx) reaction between 16 O 18 O and terminal oxygen atom of a defect TiO 2 surface, which is modeled by amorphous Ti 8 O 16 nanocluster in excited S 1 electronic state. The proposed mechanism involves four adsorption intermediates and five transition states. The computed activation energy of the POIEx equals 0.24 eV. The computed g-tensors of the predicted ozonide O 3− chemisorption species match well EPR data on O 2 adsorption on UV-irradiated nanocrystalline TiO 2 . This match serves a mean of justification of the proposed R1 mechanism of the POIEx reaction. In addition, it is found that the proposed R1 POIEx reaction's mechanism differs from R1 mechanism of thermo-assisted OIEx reaction on a surface of supported vanadium oxide catalyst VO x /TiO 2 reported earlier.