The reaction mechanisms of Br and I atoms with H2O2 have been investigated using DFT and high-level ab initio calculations. The H-abstraction and OH-abstraction channels were highlighted. The geometries of the stationary points were optimized at the B3LYP/aug-cc-pVTZ level of theory, and the energetics were recalculated with the coupled cluster theory. Spin-orbit coupling for each halogenated species was also explicitly computed by employing the MRCI level of theory. Thermochemistry for HOBr and HOI has been revised and updated standard enthalpies of formation at 298 K for HOBr and HOI are the following: Delta H-f degrees(298K)(HOBr) = (-66.2 +/- 4.6) kJ mol(-1) and Delta H-f degrees(298K)(HOI) = (-66.8 +/- 4.7) kJ mol(-1). The rate constants have been estimated using transition state theory (TST), canonical variational transition state theory (CVT), and CVT with small curvature tunneling (CVT/SCT) over a wide temperature range (250-2500 K). For the direct abstraction mechanism, the overall rate constant at 300 K was predicted to be 2.58 x 10(-16) and 7.42 x 10(-25) cm(3) molecule(-1)s(-1) for the Br + H2O2 and I + H2O2 reactions, respectively. The modified Arrhenius parameters have been estimated for the overall reactions: kBr + H2O2(T) = 4.80 x 10(-26) T-4.31 exp(-5.51 (kJ mol(-1))/RT) and k(I+H2O2)(T) = 3.41 x 10(-23) T-3.29 exp(-56.32 (kJ mol(-1))/RT).