The adsorption and reactions of the monomer and dimer of nitric acid on TiO2 rutile (110) and anatase (101) surfaces have been studied by first-principles density functional theory with ultrasoft pseudopotential approximation. The most stable configuration of HNO3 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 NO3(a) + H(a). The rotation of NO3 requires a barrier of 12.2 kcal/mol to form the didentate configuration, Ti5c−ON(O)−Ti5cH−O2c(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 2HNO3 molecules, the most stable configuration, 2(Ti5c−ON(O)OH...O2c(a)), has a structure similar to two single HNO3 adsorbates on two 5-fold coordinated Ti atoms with the adsorption energy of 12.8 kcal...