The addition of sulfides has a marked effect on the rates of onium salt induced photoinitiated cationic ring-opening polymerizations of epoxide monomers. Various behaviors have been observed that depend on the structure of the sulfide. Dialkyl sulfides strongly inhibit the photopolymerizations of these monomers, whereas diaryl sulfides have a retarding effect on the photopolymerizations. Real-time infrared spectroscopy and optical pyrometry have been employed as analytical methods to probe the kinetic effects of the addition of a variety of sulfides on cationic epoxide ring-opening photopolymerizations. A mechanism is proposed that involves the formation of sulfonium salts as intermediates. The observations made in this study have important implications for cationic photopolymelizations in general and for photoinitiated cationic ring-opening polymerizations of epoxides in particular. (c) 2005 Wiley Periodicals, Inc.