The adsorption and photooxidation of CH3CN chemisorbed on TiO2 has been investigated using infrared spectroscopy. It was found that CH3CN forms an ice layer on TiO2 at 126K and above 126K CH3CN diffusion into TiO2 occurs. At 200K, the adsorption of CH3CN on both isolated Ti-OH groups and on the Ti4+ Lewis acid site was observed. Under UV irradiation (350 +/- 50 nm; hv = 3.10-4.13 eV), CH3CN was oxidized in the presence of O-2 to form H2O, CO2, surface CO32- and an intermediate partially oxidized isocyanate species, NCO, which was identified by various isotopic experiments. Photooxidation of the CH3 moiety in adsorbed CH3CN occurs more readily does the CN moiety in CH3CN. Lattice oxygen of TiO2 was found to preferentially form adsorbed NCO, compared to oxygen from adsorbed O-2. A mechanism of the formation of NCO species was postulated.