The thermal decomposition characteristics of 2,2,6,6-tetramethyl-1-(1-phenethyloxy) piperidine, a stable covalent adduct of a benzylic radical derived from ethylbenzene and a nitroxyl radical, 2,2,6,6-tetramethyl-1-piperidinyloxy have been studied using a variety of techniques. This adduct has been used to initiate the polymerization of styrene. The adduct decomposes thermally to generate a benzylic radical capable of initiating polymerization and a passive nitroxyl radical which reversibly forms a covalent bond with the propagating chain end. Thus the polymerization proceeds under living conditions. The thermal decomposition of the adduct has been examined in detail in an attempt to better understand the reversibility of chain capture during polymerization. The adduct decomposes to form predominately styrene and a hydroxylamine corresponding to the nitroxyl fragment. Small amounts of 2,3-diphenylbutane and ethylbenzene are also formed. The activation energy for the decomposition, which reflects the cleavage of the nitrogen-oxygen bond, is 30.6 kcal/mol.