The interactions of HNO3 and NO3- with kaolinite surfaces are important in the reaction of airborne clay mineral particles with atmospheric trace gases. Theoretical calculations at the B3LYP/6-311++G(d,p) level were performed on surface clusters of HNO3 on kaolinite, on monodentate, on bidentate, and on two bridged complex structures of NO3- on kaolinite and on gas phase and solvated NO3-. The results are compatible with the ranges assigned in the literature for NO3- on mineral surfaces for the frequency of the vibrational symmetric stretches (nu(1)) and for the low-frequency branch of the asymmetric stretches (nu(3)), with slightly higher frequencies reported for the bridged models. For the higher-frequency branch of the asymmetric stretches measured on kaolinite (Angelini et al.), our calculations match the assigned values for the monodentate surface complex, while predicting an inverse assignment for the frequencies of the bidentate and bridged complexes.