The potential energy surface for the reaction of HCl with ClONO2 in the presence of NO3- has been studied using at, initio (MP2) and density functional (B3LYP) methods. The HCl + ClONO2 --> Cl-2 + HONO2 reaction is shown to have a high barrier of 45-64 kcal/mol. The NO3- catalyzes the reaction, and the entire reaction involves several associative and dissociative steps. The energetically most favorable pathway for the reaction of the complex HCl . NO3- with ClONO2 is to follow the associative steps. Here HCl ..NO3-, which is actually Cl . HNO3, can attach to ClONO2, giving the NO3ClClHNO3- complex, which, after decomposing to Cl-2 . NO3- + HNO3, forms the Cl2NO3HNO3- complex and then the observed products Cl-2 + NO3HNO3-. All the associative steps have no barrier, and the entire reaction occurs with a negative activation energy and with the overall exothermicity of 21.5 and 17.7 kcal/mol at the B3LYP and MP2 levels, respectively. If one follows dissociative steps, the reaction complex HCl . NO3- at first dissociates to give Cl- and HNO3, and in the second step, Cl- attacks ClONO2 to produce Cl-2 . NO3-, which again decomposes to Cl-2 and NO3-. These dissociative steps occur without barrier, and energetically the highest point is Cl- + HNO3. The reaction of the reactant complex ClONO2 . NO3- with HCl cannot produce Cl-2 + NO3HNO3- directly. Instead, the reaction proceeds by the formation of the NO3ClNO3HCl- complex and its decomposition to ClONO2 + Cl-HNO3 without barrier and then follows the same associative pathway to that starting with the reactant complex HCl . NO3-. Free energy calculations predict that, at low temperatures, both HCl . NO3- + ClONO2 and HCl + NO3-. ClNO3 reactions occur by the associative steps via trimolecular complexes and exhibit a negative temperature dependence of the rate coefficient.