The mechanism of the H + ClONO2 reaction is examined by performing QCISD calculations at geometries optimized at the MP2 level. Each of the six reaction channels involves stereoisomeric transition states that have identical energy barriers. The lowest energy barrier is 24.2 kcal mol(-1) for the indirect metathetical pathway leading to OH + cis-CIONO, being the corresponding rate constant calculated employing TST theory. The NO2-elimination channel and the indirect metathetical pathway leading to OH + trans-ClONO should compete with each other as they have barriers of 24.8 and 25.1 kcal mol(-1). For Cl-substitution, Cl-abstraction, and N-attack, the barriers are 27.4, 35.1, and 41.3 kcal mol(-1). (c) 2005 Elsevier B.V. All rights reserved.