In a recent article, we have showed that the nitroxide mediated polymerization of methyl methacrylate was possible up to 80% conversion for reasonable masses M-n = 60,000 g mol(-1) when 2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxyl nitroxide (DPAIO) was used as control agent. We have claimed that the success of this experiment relied on the absence of H-transfer reaction both in the alkoxyamine and between alkyl and nitroxyl radical. In this article, the decomposition of 4-nitrophenyl 2-(2,2,6,6-tetramethylpiperidine-1-yloxy)-2-methylpropionate (1a) and 4-nitrophenyl 2-(2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxy)-2-methylpropanoa te (2a) has been studied by H-1 NMR in the presence and in the absence (persistent radical effect condition) of scavenger (thiophenol PhSH). At temperature lower than the one used for polymerization, fast and quantitative H-transfer reaction was observed for la whereas no H-transfer reaction was observed for 2a. The scavenging technique proved for the first time that the H-transfer was an intermolecular process for la. However, the slow side-reaction of N-OC bond homolysis, which did not impede the control of the polymerization but may exert a detrimental effect on the livingness, was observed and quantified for 2a. (c) 2008 Wiley Periodicals, Inc.