Ab initio electronic structure computations have been performed to study the singlet and triplet potential energy surfaces involved in the gas-phase reactions of O(P-3) with the alkyl bromides C2H5Br and 1,2-C2H4Br2 to produce spin-allowed OBr and spin-forbidden HOBr products. The computations explore the effects of multiple alkane bromination on the reaction path. Our calculations indicate that high barriers to OBr formation exist on the triplet potential energy surface of alkyl-bromides reacting with O atoms. We compute spin-orbit couplings between the singlet and triplet surfaces at the lowest points where the surfaces intersect. We conclude that OBr products may be observed at sufficiently high collision energies via the triplet path while there is low probability of production of OBr or HOBr from a singlet state. (C) 2003 American Institute of Physics.