An efficient thermally latent initiation system using dual sulfonium salts, consisting S-benzylsulfonium salt 1 bearing SbF(6)(-)counter anion and S,S-dimethylsulfonium salt 2 bearing CH3 SO4- counter anion, has been developed for the cationic polymerization of epoxides. Compared to the conventional system using 1 as a thermally latent initiator, the newly developed system allowed significant improvement of stability of epoxy formulations during storage at ambient temperature without sacrificing their curability at elevated temperatures. Such a remarkable performance is attributable to the nucleophilic attack of CH3SO4- to cationic species formed unavoidably by the reaction of 1 with epoxide. Such entrapment of cationic species into the corresponding dormant led to the inhibition of undesirable chain growth of polymers during storage of epoxy formulations. In addition, the dormant can undergo dissociation at elevated temperature to give cationic species, which can readily initiate the polymerization of epoxide. (C) 2018 Wiley Periodicals, Inc.