Accurate 0 K enthalpies have been calculated for reactions of mercury with a series of small iodine-containing molecules (I-2, IBr, ICl, and IO). The calculations have been carried out with the coupled cluster singles and doubles method with a perturbative correction for connected triple excitations [CCSD(T)] using sequences of correlation consistent basis sets and accurate relativistic pseudopotentials. Corrections have been included to account for core-valence correlation, spin-orbit coupling, scalar relativity, and the Lamb shift. In a few cases coupled cluster calculations with iterative triple (CCSDT) and quadruple (CCSDTQ) excitations have been carried out to estimate the effects of higher order electron correlation. The pseudopotential calculations have also been compared to all electron calculations using second- and third-order Douglas -Kroll -Hess Hamiltonians. In addition to the reaction enthalpies, heats of formation, bond lengths, and harmonic vibrational frequencies have been calculated for the stable triatomic products HgI2, HgIBr, HgICI. and HoIO. Accurate dissociation energies, equilibrium bond lengths, and harmonic vibrational frequencies have also been calculated for each of the diatomic molecules involved in this study (HgI, HgBr, HaCl, HgO, I-2, IBr. ICl, and IO). The reported enthalpies are expected to have accuracies of 1 kcal/mol or better.