The curing behavior of bisphenol-A-type epoxide oligomers (E-P) was evaluated by differential scanning calorimetry in the presence of S-alkylsulfonium salts of dibenzothiophene, phenoxathiin, thianthrene, thioanisole, and tetrahydrothiophene as thermal latent initiators. These initiators dissolved homogeneously in E-P, except for 2,8-dimethoxy-5-methyldibenzothiophenium tetrafluoroborate, and the curing reaction of the resulting mixtures occurred on heating, except for S-methyltetrahydrothiophenium tetrafluoroborate. The initiation activity of these salts was controlled by the character of the substituents on the benzene ring, the leaving sulfide group, and the S-alkyl group. Presumably, the electron density on the sulfide moieties and the stability of the carbocation released from the sulfonium salts affected the initiating temperature. A good correlation was obtained between the initiating temperature and the electron density of the sulfur atom of the corresponding sulfides, estimated from ab initio molecular orbital calculations in which the initiating temperature became higher as the electron density of the sulfur atom increased. (C) 2001 John Wiley & Sons, Inc.