Three-component initiators generally include a light-absorbing photosensitizer, an electron donor that is often an amine, and the third component, which is usually an iodonium salt. To characterize the role of diphenyl iodonium chloride (DPI) in three-component photoinitiator systems containing methylene blue (MB) as the photosensitizer, a systematic series of electron donors was used. The Rhem-Weller equation was used to verify the thermodynamic feasibility for photo-induced electron transfer from the electron donors to the MB. Comparison of the photopolymerization rates of each two-component initiator system (containing the photosensitizer and amine) to those of the corresponding three-component system (with the addition of (DPI) allowed fundamental information regarding the role of the DPI to be obtained. It was concluded that the DPI enhances the photopolymerization kinetics in two ways: (1) it consumes an inactive MB neutral radical and produces an active phenyl radical, thereby regenerating the original methylene blue, and (2) it reduces the recombination reaction of the MB neutral radical and amine radical/cation. (C) 2004 Wiley Periodicals, Inc.