The thermochemistry of the HOSO4 radical is of crucial importance for the modeling of the atmospheric sulfur-oxidation cycle, in which HOSO4 is formed in the reaction HOSO2 + O-2 + M -> HOSO4 + M (2a). We performed high-level quantum chemical calculations to characterize the energetics of reaction (2a) and obtained the following values for the enthalpy of reaction: Delta H-R(0K) (2a) = -71.4 +/- 2.0 and Delta H-R(298K) (2a) = -72.9 +/- 3.0 kJ/mol. For the enthalpy of formation of HOSO4, it follows: Delta H-f(0K)(HOSO4) = -438.0 +/- 5.0 kJ/mol and Delta H-f(298K)(HOSO4) = -447.0 +/- 6.0 kJ/mol. These new thermochemical data can serve as a basis for kinetic studies of HOSO4 reactions in the atmosphere. (C) 2009 Elsevier B.V. All rights reserved.