The characterization and isolation of sulfurous acid (H2SO3) have never been accomplished and thus still remain one of the greatest open challenges of inorganic chemistry. It is known that H2SO3 is thermodynamically unstable. In this study, however, we show that a Ci-symmetric dimer of sulfurous acid (H2SO3)2 is 3.5 kcal mol-1 more stable than its dissociation products SO2 and H2O at 77 K. Additionally, we have investigated the kinetic stability of the sulfurous acid monomer with respect to dissociation into SO2 and H2O and the kinetic isotope effect (KIE) on this reaction by transition-state theory. At 77 K, the half-life of H2SO3 is 15 x 10(9) years, but for the deuterated molecule (D2SO3) it increases to 7.9 x 10(26) years. At room temperature, the half-life of sulfurous acid is only 24 hours; however, a KIE of 3.2 x 10(4) increases it to a remarkable 90 years. Water is an efficient catalyst for the dissociation reaction since it reduces the reaction barrier tremendously. With the aid of two water molecules, one can observe a change in the reaction mechanism for sulfurous acid decomposition with increasing temperature. The most likely mechanism below 170 K is via an eight-membered transition-state ring; yet, above 170 K, a mechanism with a six-membered transition state ring becomes the predominant one. For deuterated sulfurous acid, this change in reaction mechanism can be observed at 120 K. Consequently, between 120 and 170 K, different predominant reaction mechanisms occur for the decomposition of normal and deuterated sulfurous acid when assisted by two water molecules. However, the much longer half-life of deuterated sulfurous acid and the stability of the sulfurous acid dimer at 77 K are encouraging for future synthesis and characterization under laboratory conditions.