The photosensitized cationic polymerization of typical monomers, e.g., of oxiranes such as cyclohexene oxide and 3,4-epoxycyclohexyl-3',4'-epoxycyclohexene carboxylate, of vinyl ethers such as n-butyl vinyl ether, and of other vinyl monomers such as styrene and N-vinylcarbazole by using a dithienothiophene derivative was studied. The cationic polymerization of these monomers was initiated at room temperature upon irradiation at lambda > 350 nm either in bulk or CH2Cl2 solutions with 3,5-diphenyldithieno[3,2-b:2,3-d]thiophene (DDT) in the presence of diphenyliodonium hexafluorophosphate (Ph2I+PF6-). Photophysical properties of DDT were studied in terms of optical absorption, fluorescence, and phosphorescence spectroscopy in order to provide information that would be useful in confirming the validity of the initiation mechanism. A mechanism involving formation of exciplex and subsequent electron transfer from excited DDT to iodonium ions is proposed.