The reaction of Cl with HONO is explored theoretically using coupled cluster and Moller-Plesset perturbation theory. Various possible reaction pathways, including the H-abstraction and addition reactions, are considered. Because the rate-determining barrier in the most feasible pathway lies only 1.33/8.86 kcal/mol (Delta E-double dagger/Delta G(double dagger)), the reaction is expected to be rapid. Based on the analysis of the temperature influence of reaction mechanism, we expect that the actual reaction mechanism may vary with experimental conditions. We expect this contribution can lead us to deeply understand the mechanism of the title reaction and may be helpful for the modeling of nitrous acid-halogen combustion chemistry. (c) 2007 Elsevier B.V. All rights reserved.