The photon-assisted adsorption and catalytic decomposition of nitric oxide (NO) over undoped and Fe-doped TiO2 nanoparticles have been investigated by in situ diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy, in situ X-ray photoelectron spectroscopy (XPS) and on-line NOx analysis. The DRIFT spectra and on-line NOx analysis reveal that the usual photo-oxidation of NO to NO2 is strongly suppressed by the Fe dopant. This is found to be caused by the photo-reduction of Fe3+ to Fe2+, which is an effective adsorption site for nitric oxide species. The DRIFT spectra indeed reveal a new band at 1805 cm−1, which is assigned to the Ndouble bond; length as m-dashO stretch vibration in a Fe2+(NO)2 complex. Instead of producing NO2, photo-generated hydroxyl radicals oxidize the Fe2+ back to Fe3+. This causes the NO to desorb again, effectively closing an NO-mediated recombination loop. These results support the recently proposed reaction mechanisms for the photocatalytic decomposition of NO over undoped and Fe-doped TiO2, and provide new insights for the development of highly selective photocatalysts based on doped metal oxides.