Recently, depletion of gas-phase elemental Hg has been observed by several research groups after polar sunrise in the atmospheric boundary layer in Arctic regions. At the same time Hg compounds have been observed to accumulate in the polar snowpack. Several different oxidation reactions involving gas-phase Br and BrO have been hypothesized to explain this process. Molecular quantum mechanical methods are here applied to evaluate the energetics of such reactions, in both the gas phase and aqueous solution. The formation of HgO from the reaction of Hg-0 and BrO in the gas phase is found to be endothermic, but HgBr and HgBr2 can form exothermically through the oxidation of HgO by either Br atom radicals or Br-2. The instability of HgO has the same cause as the low stability of HgBr compared to HgBr2, i.e., the high first IP of Hg. The calculations also indicate that HgBr2 is stable photolytically, while gas-phase HgO and HgBr are decomposed by visible light.