PURPOSE: To produce the subject compd. with a small amount of by-product, easy in starting material handling and in a high yield by allowing chlorsulfonic acid to react with alkanesulfonic acid.

CONSTITUTION: Alkanedisulfonic acid, particularly methanedisulfonic acid, is obtained by allowing chlorosulfonic acid to react with alkanesulfonic acid. The reaction is executed, for example, as the reaction formula and methane sulfonic acid is allowed to react with the chlorosulfonic acid under anhydrous conditions, preferably at substantially atmospheric pressure in the presence of air or the other oxygen-containing atmosphere. The reaction temp. is 70-160°C and the reactants are heated to the range of about 100-160°C preferably. The reaction is executed in the presence of a cocatalyst such as activated carbon as a preferable method particularly.