In order to compare the mechanisms of reactions of sulfonium salts with organolithiums and Grignard reagents, the reactions between 5-substituted dibenzothiophenium salts and methyllithium, methylmagnesium halides, and phenylmagnesium halides have been investigated to get a large difference as shown in Tables I and II, respectively. The reactions of 5-substituted dibenzothiophenium salts with methyllithium gave ring-opening products (I) as main products in Table I, whereas the reactions of those with methylmagnesium iodide gave no I or only little. Instead, these reactions gave ringopening products (III) as main products in Table II. The mechanisms of the formation of the products of the runs in Table I and II have been explained as shown in Chart 2 and Chart 4, respectively.