In this communication, gamma-phenyl-gamma-butyrodithiolactone (DTL1) is presented as the first example of a new type of control agent. The styrene polymerization carried out at 60 degrees C in the presence of DTL1 exhibits living characteristics, without consuming DTL1 during the process. This unprece-dented behavior was explained by a mechanism based on the reversible formation of a persistent radical adduct between the DTL1 and the polystyrene macroradicals.