A coupled cluster composite approach has been used to accurately determine the spectroscopic constants, bond dissociation energies, and heats of formation for the X1(2)II(3/2) states of the halogen oxides ClO, BrO, and IO, as well as their negative ions ClO-, BrO-, and IO-. After determining the frozen core, complete basis set (CBS) limit CCSD(T) values, corrections were added for core-valence correlation, relativistic effects (scalar and spin-orbit), the pseudopotential approximation (BrO and IO), iterative connected triple excitations (CCSDT), and iterative quadruples (CCSDTQ). The final ab initio equilibrium bond lengths and harmonic frequencies for ClO and BrO differ from their accurate experimental values by an average of just 0.0005 angstrom and 0.8 cm(-1), respectively. The bond length of IO is overestimated by 0.0047 angstrom, presumably due to an underestimation of molecular spin-orbit coupling effects. Spectroscopic constants for the spin-orbit excited (X2II1/2)-I-2 states are also reported for each species. The predicted bond lengths and harmonic frequencies for the closed-shell anions are expected to be accurate to within about 0.001 angstrom and 2 cm(-1), respectively. The dissociation energies of the radicals have been determined by both direct calculation and through use of negative ion thermochemical cycles, which made use of a small amount of accurate experimental data. The resulting values of D-0, 63.5, 55.8, and 54.2 kcal/mol for ClO, BrO, and IO, respectively, are the most accurate ab initio values to date, and those for ClO and BrO differ from their experimental values by just 0.1 kcal/ mol. These dissociation energies lead to heats of formation, Delta H-f (298 K), of 24.2 +/- 0.3, 29.6 +/- 0.4, and 29.9 +/- 0.6 kcal/mol for ClO, BrO, and IO, respectively. Also, the final calculated electron affinities are all within 0.2 kcal/mol of their experimental values. Improved pseudopotential parameters for the iodine atom are also reported, together with revised correlation consistent basis sets for this atom.