PURPOSE: To obtain the titled addition compound useful as a sulfonating agent extremely in high yield, by reacting chlorosulfonic acid with a tertiary amine in an equimolar ratio in a hydrophobic organic solvent, bringing only the reaction product with an alkali aqueous solution.

CONSTITUTION: Chlorosulfonic acid is reacted with a tertiary amine (e.g., trimethylamine, pyridine, etc.) in an equimolar ratio in a hydrophobic organic solvent such as hexane, chloroform, carbon tetrachloride, or methyl ethyl ketone, the prepared reaction solution is brought into contact with an alkali aqueous solution (especially an aqueous solution of inorganic base is practically used) to give an addition compound of a tertiary amine and sulfur trioxide.

EFFECT: An amount of a tertiary amine used is small, and the desired compound separated and purified easily.