The sulfonium salts which contained naphtyl and α-ester alkyl groups were newly synthesized, for example, 2-naphtylmethyl-1-ethoxycarbonylethyl sulfonium hexafluorophosphate, and their properties as latent catalysts were investigated. The remarkable feature of these sulfonium salts is that they are able to work as the initiators of photo-cationic, photo-radical and thermal cationic polymerization. Because these sulfonium salts could be sensitized by thioxanthone derivatives, it was possible to apply these sulfonium salts to the pigment system, e.g. titanium dioxide contained. And this system showed the same photoactivities as the usual photo-radical initiators, and efficiently polymerized radical monomers. In the hybrid system, the reactivities of these sulfonium salts in the presence of photosensitizer were superior to those of triarylsulfonium salt (UVI-6990) and 2, 2-dimethoxy-1, 2-diphenylethane-1-one (IC-651). Further, these sulfonium salts were able to polymerize cycloaliphatic epoxides (UVR-6110) at 130°C, therefore the application of sulfonium salts to the thermal curing system was also expected in the field which required after-cure followed photopolymerization.