Surface oxygen vacancies (defects) can be created on the TiO2(110) surface by thermal annealing to high temperatures (above 500 K). Two Ti3+ sites are present at each oxygen vacancy. Adsorbed D2O, (CH2O)-C-13, or (NO)-N-15 react reductively at these Ti3+ sites to produce D-2, (C2H4)-C-13, and (N2O)-N-15, respectively. The oxygen atoms in the adsorbate molecules are preferentially extracted by the substrate TiO2 surface. The deoxygenation of adsorbates is accompanied by the oxidation of surface Ti3+ sites. The yield of reduction products (D-2, (C2H4)-C-13, and (N2O)-N-15) is therefore proportional to the coverage of surface oxygen vacancies. No deoxygenation reactions are observed on the fully oxidized (defect-free) surface. This unique property of the surface provides a convenient method to quantitatively detect the relative coverage of Ti3+ sites on the TiO2(110) surface.