13C-modified TiO2 was prepared to facilitate study of the dopant atoms and trace their chemical fate throughout the process. In the preannealed material, NMR showed strong evidence of many Ti−O−C bonds. After annealing, surface-bound coke is a major component. NMR also showed that a washing step before annealing led to the generation of orthocarbonate (C(OR)4) centers, observed at 126 ppm, which are located deep inside the TiO2 particles. Both NMR and XPS confirmed the presence of small amounts of regular sp2-hybridized carbonate species in all briefly annealed samples, while annealing for longer times led to a reduction removal of the COn centers. Quantitative NMR also shows the degree of carbon loss that accompanies annealing. Some variation in the chemical degradation of quinoline is noted among the catalysts, but coke-containing TiO2 catalysts are not qualitatively better catalysts for use with visible light with this substrate.