We investigated the formation of ground-state donor/acceptor complexes between xanthene dyes [rose bengal (RB) and fluorescein (FL)] and a diphenyliodonium (DPI) salt, which is dissolved in 2-hydroxyethyl methacrylate (HEMA) monomer. To characterize the association constant of the complex, we have suggested a new analysis model based upon the Benesi-Hildebrand model. Because the assumption of the original Benesi-Hildebrand model is that the absorption bands are only due to the presence of the complex and that the absorption by the free component is negligible, the model cannot be applied to our systems, which is a dye-based initiator system. For each dye, the molar absorptivity of the ground-state complex was evaluated as a function of wavelength, and this analysis confirmed the validity of the modified Benesi-Hildebrand model. In addition, we observed that the RB/DPI photoinitiator system failed to produce a perceptible polymerization rate but the FL/DPI photoinitiator system provided high rates of polymerization. On the basis of the association constant for these complexes, we concluded that the observed kinetic differences arise from the different association constant values of the ground-state dyeacceptor complex, resulting in back-electron transfer reaction. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1429-1439, 2009