The reaction of chlorine nitrate with hydronium ion has been examined in MP2/6-31+G* direct ab initio molecular dynamics simulations. G2MP2 energies of reactants and products have been calculated. The reaction is found to proceed directly, H3O- being attracted to ClNO3, approaching, and protonating the oxygen atom attached to Cl with no activation barrier. The reaction is exothermic. and the product (NO2+)(HOCl)(H2O) complex undergoes rearrangement and dissociation. The mechanism is similar to that observed in the reaction of nitric acid with hydronium ion. (C) 2002 Elsevier Science B.V. All rights reserved.