2-Mercaptothioxanthone (TX-SH), a hydrogen abstraction type photoinitiator for free radical polymerization, is synthesized and characterized. Its capability to act as an initiator for the polymerization of methyl methacrylate (MMA), styrene (St), and multifunctional monomers is examined. The relative efficiencies of TX-SH, the parent thioxanthone (TX), and their combination with an amine synergist such as N-methyldiethanolamine (MDEA) are compared in the photopolymerization. The postulated mechanism is based on the intermolecular reaction of triplet, (TX)-T-3-SH*, with the thiol moiety of ground-state TX-SH. Thus, when TX-SH is irradiated in the presence of a monomer, it can serve as both a triplet photosensitizer and a hydrogen donor. The resulting thiyl radicals initiate the polymerization. Incorporation of TX-S groups into polymers was demonstrated by spectroscopic methods.