The enthalpies of formation of OXO (X = Cl, Br, and I) and their anions were calculated using the Hartree-Fock, the second-order Moller-Plesset perturbation theory, the density functional theory with the B3LYP hybrid functional, and the coupled cluster theory using single and double excitation with a perturbational treatment of triplet excitation methods with two basis sets of triple-zeta plus polarization quality by employing several isodesmic (or congeneric) reactions. The weighted averages and their associated uncertainties for the enthalpies of formation were derived for these molecules using Irikura's procedure. The calculated standard enthalpies of formation at 0 K are 102.2 +/-6.5, 163.9 +/- 7.1, 113.9 +/- 10.3, -104.8 +/- 6.5, -76.0 +/- 7.0, and -135.0 +/- 10.3 kJ/mol for OClO, OBrO, OIO, OCIO-, OBrO-, and OIO-, respectively. The derived values are in excellent agreement with the available experimental values.