Accurate reaction enthalpies have been calculated using the CCSD(T) method and sequences of correlation consistent basis sets for the reactions of Hg with a series of small halogen-containing molecules (Cl-2, Br-2, BrCl, ClO, and BrO). Explicit extrapolations to the complete basis set limit are used together with accurate treatments of core-valence correlation, scalar relativity, and spin-orbit effects in order to predict both reaction enthalpies of the title reactions and heats of formation of the intermediates (HgCl2, HgBr2, HgBrCl, HgClO, and HgBrO) to an estimated accuracy of approximately 1 kcal/mol. All of the intermediates are predicted to be strongly bound and the structures and vibrational frequencies of HgClO and HgBrO are reported for the first time. The present results are expected to be useful in modeling the gas-phase oxidation of mercury in the troposphere by halogen-containing species.