The mechanism for the OH + HNO3 reaction has been studied by ab initio molecular orbital calculations at the G2M(cc3) level of theory. Four complexes and four transition states have been found and confirmed by intrinsic reaction coordinate analyses. The commonly assumed six-membered ring complex formed by hydrogen bonding of the OH radical with HNO3, -ON(O)OH...OH-, was found to be stable by 8.1 kcal/mol; its decomposition producing NO3 + H2O was predicted to have a barrier of 11.6 kcal/mol. A five-membered ring complex, -ON(O)OH...O(H)-, with the H atom of the OH radical placed out of the ring plane, was found to have a stability of 5.3 kcal/mol; it fragments to form NO3 + H2O with a barrier of 6.6 kcal/mol. Two additional complexes, which are the mirror image of each other with a 7.4 kcal/mol binding energy, were found to be related to the OH exchange reaction with a 13.3 kcal/mol barrier above the complexes. The direct abstraction process producing H2O2 and NO2 was predicted to have a large barrier of 24.4 kcal/mol, insignificant to atmospheric chemistry. The rate constant has been calculated at 200-1500 K and 0-760 Torr. The results show that the reaction has strong pressure and tunneling effects below room temperature. In addition, the rate constants for the decay of OH and OD (in OD + DNO3) evaluated by kinetic modeling compare reasonably well with experimental data below room temperature. The unusually pronounced kinetic isotope effect observed experimentally, k(H)/k(D) greater than or equal to 10, could be accounted for by the combination of the greater tunneling rate in the H system and the large redissociation rate of stabilized complexes in the D system. The rate constant predicted for the production of H2O and NO3 in the temperature range 750-1500 K can be effectively represented by the expression k = 1.45 x 10(-23) T-3.5 exp(+839/T) cm(3)/s. (C) 2001 American Institute of Physics.