The industrial synthesis entails the reaction of hydrogen chloride with a solution of sulfur trioxide in sulfuric acid:
:HCl + SO → ClSOH
It can also be prepared by chlorination of sulfuric acid, written here for pedagogical purposes as HSO(OH), vs. the usual format HSO:
:PCl + HSO(OH) → HSOCl + POCl + HCl
The latter method is more suited for laboratory-scale operations.