The geometries, vibrational spectra, and relative energetics of CH3BrO3 isomers have been examined using the B3LYP method in conjunction with various basis sets. The CH3OBrO2 isomer is found to be the lowest energy structure among the isomers, with an estimated heat of formation of 10.6 kcal mol(-1), at 0 K, as determined from G2 theory. The next lowest energy isomer is CH3OOOBr, which lies 5.6 kcal mol(-1) above CH3OBrO2. The isomers with higher energies are CH3OOBrO3 and CH3BrO3 We have examined the implication of the formation of the CH3BrO3 isomers from the atmospheric cross-reactions of the CH3O2 and BrO radical species.