A new system for efficiently carrying out cationic photopolymerizations with visible and long-wavelength UV light is described. This system is based on the principle that certain onium salt cationic photoinitiators can be reduced by free radicals produced by the hydrogen abstraction reactions of photoexcited ketones. Thus, when camphorquinone, benzil, 2-isopropylthioxanthone, and 2-ethylanthraquinone are irradiated in the presence of a monomer that can serve as a hydrogen donor, the resulting monomer-bound radical rapidly reduces a diaryliodonium salt or a dialkylphenacylsulfonium salt, and the resulting monomer-centered cations initiate the polymerizations of epoxides, vinyl ethers, and heterocyclic compounds. Onium salts with high reduction potentials, such as triarylsulfonium salts, do not undergo sensitization by this new system. (C) 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 343-356, 2001.