The geometries, vibrational spectra, and relative energetics of the HBrO3 isomers (HOOOBr and HOOBrO) and their transition states have been examined by using the quadratic configuration interaction method in conjunction with various basis sets. From the dissociation energies of the HBrO3 isomers, it is found that the most energetically favorable process during the HO2 + BrO reaction pathway is the formation of HOOBrO as an intermediate, and its eventual dissociation into HOBr and Oz, due to the very low energy barrier (2.8 kcal mol(-1)) involved. The HOOOBr species, if formed as an intermediate, will be more likely to dissociate into HBr + O-3 rather than HOBr + O-2, as the energy barrier for the latter process is quite high (26.4 kcal mol(-1)) relative to the energy barrier for the HOOOBr --> HBr + O-3 dissociation process.