The catalytic and photon-induced oxidation of NO 2 on anatase TiO 2 has been studied and compared with the surface nitrate species obtained after adsorption of HNO 3 . Using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT), and temperature-programmed desorption (TPD), it is shown that identical products are obtained in all reaction systems but that their formation rates differ significantly. The surface reaction products are identified as combinations of surface−NO 3 − species, where NO 2 bonds to the lattice oxygen, and freely adsorbed NO 3 − ions. These products can be obtained either by dissociative adsorption of HNO 3 or by catalytic/photocatalytic oxidation of NO 2 , which is facilitated by UV light. A concerted reaction mechanism is unraveled that involves reorientation of bidentate nitrate that pushes out a neighboring protonated lattice oxygen to form a surface−NO 3 − species and a terminal OH group. The thermal stability of these surface species has been studied by means of TPD and simultaneous in situ DRIFTS measurements that reveal a main desorption peak (m/z = 46) at around 430 °C, which is attributed to concerted nitrate desorption through pentoxide (N 2 O 5 ) formation. A weaker and broader TPD peak is found at about 185 °C and is attributed to desorption of nitrate species bonded in a compressed configuration. The experimental results can be explained by the changing stability of the identified nitrate products, which depends strongly on the surface chemical environment and the surface coverage. The DFT results show that the stabilization of intermediate NO 2 adsorbates and the final nitrate reaction products occurs through a bifunctional charge exchange mechanism that is mediated by the TiO 2 crystal. In particular, a stable surface−NO 3 − and NO 3 − ion pair configuration has been identified. This mechanism explains both the thermal and photoinduced oxidation of NO 2 and their thermal stability and different formation rates, yielding high photoinduced oxidation reaction rates. Our results provide insights into the structure and chemical stability of nitrate surface products on TiO 2 particles and their formation mechanism, which is important for understanding their catalyzed transformation into the harmful compounds HONO and N 2 O during continued UV light illumination.