The reaction HOSO(2)+O(2)-->HO(2)+SO(3) (2) is of crucial importance for sulfuric acid formation in the atmosphere, and reliable thermochemical data are required for an adequate modeling. The currently least well known thermochemical quantity of reaction (2) is the enthalpy of formation of the hydroxysulfonyl radical (HOSO(2)). We report on high-level quantum chemical calculations to predict the binding energy of the HO-SO(2) bond and deduce a value for the enthalpy of formation of HOSO(2) using the most reliable thermodynamic data of OH and SO(2). On the basis of anharmonic vibrational frequencies from calculations at the fc-CCSD(T)/cc-pV(T+d)Z level of theory, the enthalpy of reaction at 0 K for the reaction OH+SO(2)-->HOSO(2) (1) was computed to be Delta(R)H(0 K)(1)=-109.4+/-2.0 kJ/mol and the thermal corrections result in Delta(R)H(298 K)(1)=-114.7+/-3.0 kJ/mol. From these values, we obtain Delta(f)H(0 K)(HOSO(2))=-366.6+/-2.5 and Delta(f)H(298 K)(HOSO(2))=-374.1+/-3.0 kJ/mol, respectively. Accordingly, Delta(R)H(0 K)(2)=-8.5+/-3.0 and Delta(R)H(298 K)(2)=-9.5+/-3.0 kJ/mol.