The mechanism for the formation of the oxidant peroxymonocarbonate from hydrogen peroxide and bicarbonate/carbon dioxide in aqueous solution is studied by electron structure- and statistical mechanics techniques. It is found that the reaction proceeds through a single transition state. The activation energy is 83.3 kJ/mol at the highest level of theory [CCSD(T)/aug-cc-pvdz//B3LYP/aug-cc-pvdz]. However, it is reduced by 17% when the counter ion (viz. NH4+) is incorporated in the calculations. (c) 2007 Elsevier B.V. All rights reserved.