Despite the key role of the HOSO radical in the combustion of sulfur-rich fuels, the thermochemistry of this simple species is not well-established. Due to the extraordinary sensitivity of the potential energy surface to basis set and electron correlation methods in ab initio computations, there is no consensus in the literature regarding the structure of the global minimum syn-HOSO. A definitive enthalpy of formation for HOSO is presented, based on systematically extrapolated ab initio energies, accounting for electron correlation primarily through coupled cluster theory, including up to single, double, and triple excitations with a perturbative correction for connected quadruple excitations [CCSDT(Q)]. These extrapolated valence electronic energies have been corrected for core-electron correlation, harmonic and anharmonic zero-point vibrational energy, and non-Born-Oppenheimer and scalar relativistic effects. Our final recommended enthalpy of formation is Delta H-f(0)degrees(syn-HOSO) = -58.0 kcal mol(-1). The planar anti-HOSO transition state lies 2.28 kcal mol(-1) above the syn-HOSO minimum, while predicted reaction enthalpies for H + SO2 -> HOSO, HOSO -> OH + SO, HOSO + H -> H-2 + SO2, and OH + HOSO -> SO2 + H2O are -38.6, 68.0, -64.4, and -80.1 kcal mol(-1), respectively. We provide incontrovertible evidence for a quasi-planar structure of the syn-HOSO radical, with a remarkably flat torsional energy surface, based on CCSD(T) geometries and harmonic vibrational frequencies energies with up to quintuple-zeta quality basis sets. The energy separation between planar syn-HOSO and the nonplanar global minimum is a mere 5 cm(-1) at the cc-pV(T+D)Z CCSD(T) level of theory. Computed fundamental vibrational frequencies for syn-HOSO and syn-DOSO based on a full quartic force-field evaluated at the cc-pV(T+d)Z CCSD(T) level of theory are in agreement with available experimental data. The present results confirm a previously tentative assignment of a band at 1050 cm(-1) to the HOS bending mode.