The anthracene-sensitized photodecomposition of diphenyliodonium hexafluorophosphate was studied in an epoxy matrix. From steady-state photolysis, the generation of protons, which are considered to be the actual initiators of the polymerization of epoxides, was confirmed. In addition, 9-phenylanthracene was detected as a main product from anthracene. From time-resolved laser-flash photolysis, a broad absorption band with a peak at about 500 nm was observed that was thought to be due to the precursor of 9-phenylanthracene. On the basis of these results, we propose electron transfer from anthracene in the excited singlet state to the diphenyliodonium. cation as the initial step of photoacid generation. This process is followed by fast chemical reactions, which generate 9-phenylanthracene and protons, and back electron transfer from the diphenyliodine radical to the anthracene cation radical.