The adsorption and reactions of the monomer and dimer of nitric acid on TiO 2 rutile (110) and anatase (101) surfaces have been studied by first-principles density functional theory with ultrasoft pseudopotential approximation. The most stable configuration of HN0 3 on the rutile surface is a molecular monodentate adsorbed on the 5-fold coordinated Ti atom with the hydrogen bonded to a neighboring surface bridging oxygen with the adsorption energy of 6.7 kcal/mol. It can dissociate its H atom to a nearest bridged oxygen with almost no barrier to produce NO 3 (a) + H(a). The rotation of NO 3 requires a barrier of 12.2 kcal/mol to form the didentate configuration, Ti 5c - ON(O)-Ti 5c H-O 2c (a), which adsorbs on two 5-fold coordinated Ti atoms with the adsorption energy of 16.5 kcal/mol. In the case of the adsorption of 2HNO 3 molecules, the most stable configuration, 2(Ti 5c - ON(O)OH...O 2c (a)), has a structure similar to two single HNO 3 adsorbates on two 5-fold coordinated Ti atoms with the adsorption energy of 12.8 kcal/mol, which is about twice that of the single HN0 3 molecule. The result suggests that the interaction of the two planar HN0 3 adsorbates is negligible. The dehydration from 2(Ti 5c - ON(O)OH...O 2c (a)) forming N 2 O 5 (a) + H 2 O(a) requires an energy barrier of 46.2 kcal/mol, indicating that the dimerization of the two HNO 3 (a) is difficult. Similar adsorption phenomena appear on the anatase (101) surface. In addition, we find that the coadsorption of hydrogen plays a significant role in the adsorption energies of adsorbates, especially for the NO 3 radical, which may be employed as a linker between semiconductor quantum dots such as InN and the TiO 2 surface.