PURPOSE: To prepare an aromatic sulfinic acid, easily, in high yield, without environmental pollution and without using toxic copper catalyst, by reacting an aromatic diazonium salt with sulfur dioxide gas in the presence of activated charcoal.

CONSTITUTION: An aromatic diazonium salt such as an aniline diazonium salt prepared from aniline, sulfuric acid, sodium nitrite and water, is reacted with sulfur dioxide gas in the presence of pref. 0.5W2 times volume (based on the aromatic diazonium salt) of activated charcoal, e.g. at -10W+15°C. The resulted aromatic sulfinic acid is separated and purified by concentration, filtration, etc.