The gas-phase reaction between HOSO and NO2 was examined using density functional theory. Geometry optimizations and frequency computations were performed at the B3LYP/6-311++G-(2df,2pd) level of theory for all minimum species and transition states. The ground-state potential energy surface, including activation energies and enthalpies, were calculated using the ab initio CBS-QB3 composite method. The results suggest that the addition of HOSO and NO2 leads to two possible intermediates, HOS(O)NO2 and HOS(O)ONO, without any energy barrier. The HOS(O)NO2 easily decomposes into HONO + SO2 through the product complex HONO center dot center dot center dot SO2, whereas the HOS(O)ONO dissociates to HOSO2 + NO products. This latter dissociation is from isomerization of the HOS(O)ONO to HOS(NO)O-2. Also HOS(O)NO2 isomerization to HOS(O)ONO is hindered due to the presence of a large energy barrier. From the thermodynamic aspect the main products in the title reaction are HONO + SO2, whereas HOSO2 + NO are expected as a minor products.