High-purity HSO3Cl is prepd. at a high space-time yield by mixing and reacting gaseous HCl with SO3 dissolved in ClSO3H, contg. 0.3-1.4% SO3 at 70-100 Deg in a venturi tube at 1-1.1 mol HCl/mol SO3. Thus, into the throat of a vertical venturi tube was fed gaseous HCl at 520 L/h and a rising stream of a soln. of SO3 in HSO3Cl. The reaction mixt. at 80 Deg then passed into the bubble column above the venturi where the reaction was completed, excess HCl being discharged at the top of the column, and HSO3Cl withdrawn through a side outlet and cooled. The product was collected at an outlet at 2.9 kg/h. The remaining CSO3Cl was recycled to the venturi reactor after the addn. of 1.05 L SO3/h. [on SciFinder (R)]