Abstract: A process for producing a sulfonium compound of the formula (III) ##STR1## which process comprises reacting a p-dialkylsulfoniophenol of the formula (I) ##STR2## and a carbonyl halogenide compound of the formula (II) ##STR3## in the presence of a secondary amine having a structure in which two secondary alkyl groups are linked to the nitrogen atom, wherein R.sup.1 and R.sup.2 are the same or different and are independently an alkyl group having 1 to 4 carbon atoms, X.sup.1 is a hydrogen atom, a halogen atom or an alkyl group having 1 to 4 carbon atoms, Y.sup.- is a halogen anion, a perchlorate anion, an alkylsulfate anion, a hydrogensulfate anion or a p-toluenesulfonate anion, R is an alkyl group having 1 to 20 carbon atoms, a tert-butoxy group, a benzyloxy group, a p-methoxybenzyloxy group, a phenyl group or a 9-fluorenylmethoxy group, and X.sup.