The unimolecular chemistry of the system [HOOH]H+ has been investigated usi
ng ab initio quantum chemical methods. In analogy with the isoelectronic sy
stems-[H2NNH2]H+ and [HONH2]H+, in this paper subject to more detailed stud
ies than previously-the lowest energy pathway for decomposition of protonat
ed hydrogen peroxide is loss of an oxygen atom in its triplet electronic st
ate, giving H3O+ as the ionic product. This process requires a crossover fr
om the singlet to the triplet potential energy surface, and the minimum ene
rgy crossing point was located. The proton affinity was also calculated and
found to be in good accordance with one experimental determination.