Three-component systems, which contain a light-absorbing species (typically a dye), an electron donor (typically an amine), and a third component (usually an iodonium salt), have emerged as efficient, visible-light-sensitive photoinitiators. Although three-component systems have been consistently found to be faster and more efficient than their two-component counterparts, these systems are not well understood and a number of distinct mechanisms have been reported in the literature. In this contribution, photodifferential scanning calorimetry and in situ, time-resolved, laser-induced, steady-state fluorescence spectroscopy were used to study the initiation mechanism of the three-component system methylene blue, N-methyldiethanolamine and diphenyliodonium chloride. Kinetic studies based upon photodifferential scanning calorimetry reveal a significant increase in polymerization rate with increasing concentration of either the amine or the iodonium salt. However, the laser-induced fluorescence experiments show that while increasing the amine concentration dramatically increases the rate of dye fluorescence decay, increasing the DPI concentration actually slows consumption of the dye. We concluded that the primary photochemical reaction involves electron transfer from the amine to the dye. Pie suggest that the iodonium salt reacts with the resulting dye-based radical (which is active only for termination) to regenerate the original dye and simultaneously produce a phenyl radical (active in initiation) derived from the diphenyliodonium salt. O 2000 John Wiley & Sons, Inc.