S-Methylsulfonium salts, obtained by desulfurization of vacuum gas oil (VGO) and catalytic-cracked gasoline (CCG) based on alkylation and a subsequent precipitation process, were used as initiator for polymerization of epoxide (glycidyl phenyl ether: GPE). The initiation behavior and activity for the salts obtained from VGO and CCG were compared with those obtained from light oil. Both the salts from VGO and CCG showed almost no activity at room temperature but become active with an increase of temperature, thus indicating that the salts act as thermal latent initiator. The initiation activity for the salts from VGO was relatively low, but the salts from CCG showed significantly higher activity than the salts from light oil. Most of the salts, obtained from CCG, are derived from less nucleophilic thiophenes, and therefore unstable against thermal stimulation, thus giving the higher initiation activity.