Photolysis of tetrasubstituted bi(phosphine sulfides) at 254 nm yields disubstituted thiophosphinyl radicals that initiate the polymerization of methyl methacrylate. The phosphorus-containing chain ends of the polymers produced in these polymerizations were easily measured and characterized using P-31-NMR spectroscopy. These measurements showed that thiophosphinyl radical additions to methyl methacrylate are regiospecific. The yields obtained in bi(phosphine sulfide)-initiated MMA polymerizations compare favorably with yields obtained using a commercially available photoinitiator, Lucirin TPO, but the reaction times required are considerably longer. The molecular weights of PMMA samples obtained from bi(phosphine sulfide)-initiated polymerizations are lower than those of TPO-initiated samples, due to the ability of bi(phosphine sulfides) to also act as chain transfer agents.