Novel sulfonium salts [methyl-, 2-indany-, or 1-ethoxycarbonylethyl methyl-2-naphthylsulfonium hexafluorophosphate and 2-indany-, 1-ethoxycarbonylethyl-, 2-methyl-2-phenylpropyl-, 2-phenylpropyl-, 2-phenylethyl-, 2-(4-methoxyphenyl)-ethyl-, or 3-(4-methoxyphenyl)-2-propyl methylphenylsulfonium hexafluorophosphates] were synthesized by the reaction of dimethylsulfate and the corresponding sulfides followed by anion exchange with KPF6. These sulfonium salts could polymerize epoxy monomers at lower temperatures than previously reported for benzylsulfonium salt initiators. In particular, sulfonium salts with naphthyl groups showed higher photoactivity than already reported for di(4-tert-butylphenyl)iodonium and triphenylsulfonium hexafluorophosphates. These sulfonium salts showed higher activity in photoradical polymerization and photocationic polymerization. The photopolymerization was accelerated by the addition of 4-methoxy-1-naphthol, N-ethylcarbazole, 2,4-dimethylthioxanthone, phenothiazine, and 2-ethyl-9,10-dimethoxyanthracene as photosensitizers.