We report the first theoretical study of noncovalent and covalent interactions in formic acid (FA)-SO2 complexes. Using ab initio and DFT model chemistries, five stable noncovalent complexes were identified, as well as a covalent adduct, formic sulfurous anhydride HOSO2CHO. syn-FA is predicted to form two nonplanar bidentate complexes with SO2: the more stable one contains a normal hydrogen bond donated by OH, and the less stable one contains a blue-shifted hydrogen bond donated by CH. Both are stabilized by charge transfer from FA to SO2. anti-FA forms three planar complexes of nearly equal energy containing OH-to-SO2 hydrogen bonds. Formic sulfurous anhydride forms via an endothermic concerted cycloaddition. Natural bond orbital analysis showed that the bidentate SO2-FA complexes are stabilized by n ->pi* donation from FA to SO2, and back-donation from SO2 n and pi* orbitals into FA sigma(OH)* or sigma(CH)* orbitals. The bidentate formic acid-SO2 complex that contains an O-H center dot center dot center dot O hydrogen bond is more stable than the similar nitric acid-SO2 complex. The latter contains a stronger hydrogen bond but shows no O -> S charge transfer interaction.