The cationic initiation activity of derivatives of S-methylsulfonium salts of dibenzothiophene (3a), diphenyl sulfide (4a), thioanisole (4d), and tetrahydrothiophene (5) was evaluated in the polymerization of glycidyl phenyl ether (1). These initiators mere soluble in 1 and capable of initiating the cationic polymerization of 1 on heating, with the exception of methyltetrahydrothiophenium tetrafluoroborate (5; in the range of room temperature to 160 degrees C). Among them, methyldiphenylsulfonium tetrafluoroborate (4a) showed a moderate thermal latency that brought about the polymerization of 1 efficiently at 160 degrees C but not below 80 degrees C. S-Alkylsulfonium salts of aromatic sulfides such as phenoxathiin (6a) and thianthrene (6b) also mere evaluated for their activity in the cationic polymerization of 1, fi om which the thermal latent behavior of these salts also was confirmed (i.e., there was no reaction at 60 degrees C for 3 h, but there was a high enough conversion at 140 degrees C). Furthermore, the catalytic activity of S-alkylsulfonium derivatives was controllable by both the property of the substituents on the aromatic rings and the character of the alkyl groups on the sulfur atom.