The reaction of a Criegee intermediate (CI) with SO 2 is of significant interest due to its negative impact on the environment and ecosystem. In the present work, the formation of H 2 SO 3 through the reaction of the CIs CH 2 OO, syn-CH 3 CHOO, and (CH 3 ) 2 COO with SO 2 is studied by using high-level electronic structure calculations. The reaction of CIs with SO 2 is a stepwise process; in the first step an O atom from a CI is transferred to the S atom of SO 2 , which results in the formation of SO 3 through a heterozonide (HOZ) formation in a barrierless reaction, and in the next step, the two hydrogen atoms of a CI are transferred to SO 3, resulting in the formation of H 2 SO 3 and other carbon-containing compounds. The calculated thermochemical parameters associated with the formation of intermediates and products show the spontaneity of the reactions. The rate constant calculated for the studied reactions shows that the formation of sulfurous acid through the reaction of CH 2 OO with SO 2 is faster than the reaction of syn-CH 3 CHOO or (CH 3 ) 2 COO with SO 2 . Our results show that the reaction of CIs with SO 2 is one of the significant processes in the atmosphere and is responsible for the formation of H 2 SO 3 , carbon monoxide, ketene, cyclopropanone, 2-oxopropyl hydrogen sulfite, and other sulfur-containing compounds in the atmosphere.