A series of 1,3,5-trithiane derivatives, including alpha- and beta-isomers of the methyl and phenyl derivatives, was investigated for use as coinitiators in benzophenone-induced photopolymerizations. To study the mechanism of the photoinduced production of initiating radicals, nanosecond laser flash photolysis was used. The rate constants for quenching the benzophenone tripler state by various trithianes and the associated quantum yields of ketyl-radical formation were determined. Photopolymerizations were carried out with a multifunctional methacrylate model monomer, and the progress of the polymerizations was followed by differential scanning calorimetry. The resulting plots showing the progress of the rates of polymerization and the conversions of double bonds were compared to the photochemical information for the various trithianes. It was found that the polymerization efficiency of the coinitiators did not follow the efficiency of photoinduced formation of the initiating radicals. The reasons for this lack of correlation were discussed in terms of how the hydrogen-donating ability of coinitiators and the reactivity of the radicals formed affected the various stages of the polymerization. Further studies with a cleavage-type photoinitiator in the presence of the trithianes lent supporting evidence for the role that the trithiane radicals play in the polymerization of the benzophenone/trithiane systems investigated in this work.