Effect of coinitiator structure on relative initiation efficiency of two-component Eosin/coinitiator systems has been evaluated quantitatively in polymerization of poly(ethylene glycol) diacrylate as an example monomer. The initiation efficiency has been measured by the Fluorescence Probe Technique (FPT), using Eosin both as a photoinitiator component and as a fluorescent probe. A LED/fiber optic-based measurement system has been developed and applied in this study. It has been found that from among 17 compounds tested, the following coinitiators form most efficient photoinitiating systems in combination with Eosin, when exposed to visible light: coinitiator, relative efficiency = triethanolamine, 1.0; 2,6-diisopropyl-N,N-dimethylaniline, 0.70; 2-benzyl-2-(dimethylamino)-4'-morpholinobutyrophenone (Irgacure 369), 0.91; carbon tetrabromide, 2.1; [4-[(2-hydroxytetradecyl)oxy]phenyl]phenyliodonium hexafluoroantimonate (SarCat 1012), 28. These relative efficiencies refer to the following component concentrations: [Eosin] = 8.6 x 10(-4) M, (0.05% by weight); [coinitiator] = 4.3 x 10(-2) M. The factors affecting the initiation efficiency of the systems studied are discussed. (C) 2008 Wiley Periodicals, Inc.