The isomerization reaction HOI -> HIO has been investigated using quantum mechanical techniques. The activation energy of the reaction at the CCSD(T) level of theory is 77.0 kcal/mol. Molecular parameters and relative energies of HOI, HIO, five HOI-H2O complexes, three HIO-H2O complexes and four transition states have been calculated by the B3LYP method. For isomerization. assisted by a single water molecule, the activation energy whereby the water molecule directly facilitates proton transfer during isomerization, reduces to 48.4 kcal/mol. Where the water molecule interacts with the HOI/HIO system by forming hydrogen-bond and/or van der Waals complexes, the activation energy is not significantly reduced. (c) 2007 Published by Elsevier B.V.