S-Methylsulfonium salts obtained by a desulfurization process for light oils, based on a methylation of sulfur-containing compounds with CH3I and AgBF4 followed by precipitation, have been used as an initiator for cationic ring-opening photopolymerization of epoxide (glycidyl phenyl ether, GPE). The sulfonium salts obtained from light oils demonstrate higher initiation activity than the salts of pure dibenzothiophenes (DBTs) and benzothiophenes (BTs). These salts are derived from highly alkyl-substituted DBTs and BTs, which have a higher molar extinction coefficient and have a sulfur atom of lower nucleophilicity than the pure DBTs and BTs. These salts are, therefore, easily photoexcited and eliminate the S-methyl group, thus resulting in higher initiation activity. The obtained findings demonstrate potential use of the sulfonium salts derived from light oils as an initiator for cationic photopolymerization.