The known diatomic halogen oxides XO (X = I-F) are studied using the all-electron QCISD and QCISD(T) methods of electronic structure theory with the 6-311+G(3df) basis set. From extensive comparisons to the available ground-state spectroscopic data on XO, these methods are found to give usefully accurate values of the molecular constants, vibrationally averaged electric dipole moments, fundamental vibrational transition moments (mu(01)), and absolute infrared absorption intensities (S-01(T)) of IO, BrO, and ClO, although certain difficulties describing FO are encountered. The all-electron, QCISD(T)-based G2 and G2(QCI) methods are used to calculate dissociation energies (Do) and adiabatic electron affinities (EA(0)) and ionization energies (IE(0)). While the G2(QCI) energies are in reasonable accord with all of the well-determined spectroscopic values, some of the G2 EA(0)(XO) are not because of reinforcing basis set additivity errors. Halogen group trends in the physical properties of XO (X not equal At) are highlighted, and predictions of mu(01) = -0.008 D, S-01(298) = 0.4 cm(-2) atm(-1), and IE(0) = 9.8 eV for IO and r(e) = 1.806 Angstrom for IO+ are made. Using G2(QCI)-like energies, we have determined the ideal gas heat of formation of IO, Delta H-f degrees(0), to be 31.0 +/- 1.0 kcal mol(-1). Thermodynamic data for IO are derived in the 0-350 K range and used to compute various reaction energies of importance to atmospheric chemistry, which are briefly discussed. Also derived are the Do = 37.9 +/- 1.2 kcal mol(-1) and Delta H-f degrees(0) = -23.8 +/- 1.1 kcal mol(-1) of IO- and the D-0 = 68.7 +/- 2.2 kcal mol(-1) and Delta H-f degrees(0) = 257.0 +/- 2.2 kcal mol(-1) of IO+.