AbstractIn order to study the thiyl radical, S.S′‐Diphenyldithiocarbonate (DTC) was used as a photo‐sensitizer of radical polymerization of styrene, methyl methacrylate and acrylonitrile, and the rate of decomposition of DTC was also studied. DTC was decomposed into phenyl thiyl radical and carbon monoxide by ultra‐violet light, but it was not decomposed by heat at 60°C. The thiyl radical which derived from the photo‐dissociation of DTC initiated the radical polymerization, and also worked as a terminator or a chain transfer agent.The kinetic of photo‐sensitized polymerization by DTC coincided with the ordinary radical polymerizati