A study of the photoinitiated and thermally initiated cationic polymerizations of several monomer systems with S,S-dialkyl-S-(3,5-dimethylhydroxyphenyl)sulfonium salt (HPS) photoinitiators bearing different lengths of alkyl chains on the positively charged sulfur atom has been conducted. HPS photoinitiators are capable of photoinitiating the cationic polymerization of a wide variety of epoxy and vinyl ether monomers directly on irradiation with short-wavelength UV light. Aryl ketone photosensitizers are effective in extending the spectral response of these photoinitiators into the long-wavelength UV region. Kinetic studies with real-time infrared spectroscopy show that HPS photoinitiators exhibit good efficiency in the polymerization of epoxide and vinyl ether monomers. Comparative studies also demonstrate that S,S-dimethyl-S-(3,5-dimethyl-2-hydroxyphenyl)sulfonium salts are more active photoinitiators than their isomeric S,S-dimethyl-S-(3,5-dimethyl-4-hydroxyphenyl)sulfonium salt counterparts. Both types of HPS photoinitiators display reversible photolysis as a result of facile termination reactions that take place between the growing chains ends with the photogenerated sulfur ylides. Preliminary studies have shown that HPS photoinitiators can also be employed as thermal initiators for the cationic ring-opening polymerization of epoxides at moderate temperatures. (C) 2003 Wiley Periodicals, Inc.