The mechanism for production of singlet oxygen from hydrogen peroxide and hypochlorite in aqueous solution is studied computationally. Geometries of various complexes and transition states are found by quantum mechanical geometry optimizations of the species in gas phase. The activation energies pertaining to the mechanism in aqueous solution are estimated using the isodensity polarized continuum method at various levels of theory up to CCSD//B3LYP/6-311+G(d,p). Three reaction paths have been identified: two with a single transition state and one involving two transition states and a H2O3 intermediate. The latter path has the smallest activation energy. (c) 2006 Elsevier B.V. All rights reserved.