ysis to form a sulfinyl carboxylate, or a salt thereof, of formula (III): ##STR16## in which X is an alkali metal, Y is hydrogen or a metal, or X and Y together represent a divalent alkaline earth metal; and decarboxylating the sulfinyl carboxylate of formula (III) to form the sulfoxide compound of formula (I), the groups R, R1, and R2 in formulae (II) and (III) having the same meanings as given a