The structures, vibrational spectra, and relative energetics of HBrO3 isomers and the transition states for their isomerization have been examined using the quadratic configuration interaction method in conjunction with different basis sets. The calculated energetics for the dissociation and isomerization pathways of HBrO3 show that the energy barriers for the isomerization of HOOOBr to HOOBrO (26.7 kcal mol(-1)) and of HOOBrO to HOBrO2 (22.2 kcal mol(-1)) are large enough to prevent such isomerization from occurring. Thus, the reaction between HO2 and BrO radicals will proceed through the exclusive formation of HOOBrO to produce HOBr and O-2. There is a possibility of the HOOOBr species being formed as well, but if this happens, HOOOBr will not isomerize to HOOBrO.