Thermal reactions of the alkoxyamine diastereomers DEPN-R' [DEPN: N-(2-methylpropyl)-N-(1-diethylphosphophono-2,2-dimethyl-propyl)-aminoxyl- R' : methoxy-carbonylethyl and phenylethyl] with (R,R) + (S,S) and (R,S) + (S,R) configurations have been investigated by H-1 NMR at 100 degreesC. During the overall decay the diastereomers interconvert, and an analytical treatment of the combined processes is presented. Rate constants are obtained for the cleavage and reformation of DEPN-R' from NMR, electron spin resonance, and chemically induced dynamic nuclear polarization experiments also using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) as a radical scavenger. The rate constants depend on the diastereomer configuration and the residues R'. Simulations of the kinetics observed with styrene and methyl methacrylate containing solutions yielded rate constants for unimeric and polymeric alkoxyamines DEPN-(M)(n)-R'. The results were compatible with the known DEPN mediation of living styrene and acrylate polymerizations. For methyl methacrylate the equilibrium constant of the reversible cleavage of the dormant chains DEPN-(M-n)-R' is very large and renders successful living polymerizations unlikely. Mechanistic and kinetic differences of DEPN- and TEMPO-mediated polymerizations are discussed.