The reaction of nitric acid with hydronium ion has been examined in MP2/6-31+G* ab initio direct molecular dynamics (MD) simulations. Four transition state structures and their G2MP2 energies have been obtained, and they are compared to energies of reactants, products and intermediates. The reaction is found to proceed by a series of intermolecular proton transfers, usually ending in dissociation of the products. HNO3(H3O+) reactive intermediates were found to play significant roles in the mechanism, and the dominant reaction channel is HNO3+ H3O+ --> NO2+ (H2O) + H2O