The reaction IO + NO -> I + NO2 has been studied theoretically using DFT and ab initio methods. Six transition state configurations have been characterized in total. Four of them correspond to the conformational and isomerization barriers among the reaction intermediates, namely between cis-and trans-iodine nitrites, IONOc, IONOt and nitryl iodide, INO2. Interesting features identified in the potential energy surface are the two distinct iodine abstraction transition states that lead to I + NO2 through each nitrite conformer. The heats of formation for the three bound intermediates are evaluated based on the calculated dissociation energies and are compared to the corresponding properties of the Cl and Br derivatives. (c) 2006 Elsevier B.V. All rights reserved.