Its structure was debated for many decades until in 1941 Shrinivasa Dharmatti proved by magnetic susceptibility that chlorine is directly bonded to sulfur.

Upon standing with excess sulfur trioxide, it decomposes to pyrosulfuryl chlorides:[7] The industrial synthesis entails the reaction of hydrogen chloride with a solution of sulfur trioxide in sulfuric acid:[7] It can also be prepared by the method originally used by acid's discoverer Alexander William Williamson in 1854,[4] namely chlorination of sulfuric acid, written here for pedagogical purposes as HSO3(OH) vs. the usual format H2SO4: The latter method is more suited for laboratory-scale operations.

[8] ClSO2OH is used to prepare alkyl sulfates, which are useful as detergents and as chemical intermediates:[7] One historical synthesis of saccharin begins with the reaction of toluene with ClSO2OH to give the ortho- and para-toluenesulfonyl chloride derivatives: Oxidation of the ortho isomer gives the benzoic acid derivative that then is cyclized with ammonia and neutralized with base to afford saccharin.

Chlorosulfonic acid has been used as an anti-contrail agent in Ryan Model 147 reconnaissance drones,[9] and to produce smoke screens.

[10][11] ClSO3H reacts violently with water to yield sulfuric acid and hydrogen chloride, which are corrosive: