The thermal decomposition of five alkoxyamines labeled TEMPO-R, where TEMPO was 2,2,6,6-tetramethylpiperidinyl-N-oxyl and R was cumyl (Cum), 2-tertbutoxy-carbonyl-2-propyl (PEst), phenylethyl (PhEt), 1-tert-butoxy-carbonylethyl (EEst), or 1-methoxycarbonyl-3-methyl-3-phenylbutyl (Acrylate-Cum), was studied with H-1 NMR in the absence and presence of styrene and methyl methacrylate. The major products were alkenes and the hydroxylamine 1-hydroxy-2,2,6,6-tetramethyl-piperidine (TEMPOH), and in monomer-containing solutions, unimeric and polymeric alkoxyamines and alkenes were also found. Furthermore, the reactions between TEMPO and the radicals EEst and PEst were studied with chemically induced dynamic nuclear polarization. In comparison with coupling, TEMPO reacted with the radicals Cum, PEst, PhEt, and EEst and their unimeric styrene adducts by disproportionation to alkenes and TEMPOH only to a minor extent (0.6-3%) but with the radical adducts to methyl methacrylate to a considerable degree (greater than or equal to 20%). Parallel to the radical cleavage, TEMPO-EEst (but not the other alkoxyamines or TEMPO-Acrylate-Cum) underwent substantial nonradical decay. The consequences for TEMPO-mediated living radical polymerizations are discussed.