The photosensitization mechanism for cationic polymerizations initiated by diaryliodonium salts photosensitized by anthracene was investigated using fluorescence and phosphorescence spectroscopy. In situ photosensitizer fluorescence measurements confirmed that the photosensitization reaction proceeds by an electron transfer process. Transient phosphorescence studies demonstrated that electron transfer occurred from the triplet excited state of anthracene to the initiator, with an intrinsic kinetic rate constant of 2 X 10(8) L/mol s. Further evidence for the role of the triplet state was provided by an observed seven-fold decrease in the polymerization rate upon addition of a tripler state quencher. Finally, numerical solution of the photophysical kinetic equations indicated that the triplet state concentration was approximately three orders of magnitude higher than that of the singlet state, and that 94-96% of the active cationic centers are produced by reaction of the initiator with the tripler state. These results indicate that the electron transfer occurs primarily from the triplet state of anthracene, with the singlet state providing only a minor contribution to the photosensitization reaction.