The possible reactions of HO2 radical with the intermediates of the Cl2SO photolysis (ClSO and SO) were studied using G3MP2//B3LYP/cc-pVTZ+d level of theory and Martin's W1U method. For the reaction between HO2 and ClSO radicals, the following mechanisms are supposed to be the main reaction pathways HO2 + ClSO --> 3HOO x ClSO --> HOO(Cl)SO --> OH + ClSO2 --> HO + Cl + SO2, HO2 + ClSO --> 3HOO x ClSO --> HOO(Cl)SO --> OH + ClSO2 --> HO(Cl)SO2. On the basis of G3MP2//B3LYP/cc-pVTZ+d and highly accurate W1U calculations, the reaction of HOO with 3SO species has also been explored, and the following dominant consecutive reactions may describe the fast oxygen transfer HO2 + 3SO --> 4HOO x SO --> 2HOOSO --> OH + SO2. In both reaction mechanisms, the first step is a barrierless formation of relatively stable van der Waals complexes that lead via intersystem crossing to intermediate adducts. Thermodynamically favored decomposition products of 2HOOSO are OH radical and SO2. In the case of the ClSO and HO2 reaction, the dissociation of HOO(Cl)SO resulted in OH and ClSO2. Further decomposition of ClSO2 to Cl atom and SO2 competes with formation of HO(Cl)SO2 via OH addition reaction to ClSO2. We also report on high-level quantum chemical calculation (W1U) to predict values for the heat of formation of 2HSO3, 2HOOSO, and 2OOS(H)O radicals using the most reliable thermodynamic data of OH and SO3: Delta(f)H(298.15K)(2HSO3) = -256.2 kJ/mol, Delta(f)H(298.15K)(2HOOSO) = -152.6 kJ/mol, and Delta(f)H(298.15K)(2OOS(H)O) = -8.3 kJ/mol. On the basis of W1U standard reaction enthalpy for the reaction ClSO + HOO --> HCl + SO3, the heat of formation for the ClSO radical was also computed to be Delta(f)H(298.15K)(ClSO) = 102.6 kJ/mol within 4 kJ mol(-1) error.