A straightforward synthesis of new seven- and eight-membered cyclic alkoxyamines from the corresponding lower homologous keto-alkoxyamines via ring-enlargement using TMS-diazomethane is described. The use of these ring-enlarged cyclic alkoxyamines as regulators/initiators for the radical polymerization of styrene and n-butyl acrylate is presented. Efficient controlled and living styrene polymerization (molecular weight of up to 40 000) can be obtained using the seven- and eight-membered alkoxyamine initiators. The influence of the ring-enlargement on the quality of the polymerization process (polymerization time, livingness, PDI) is discussed. The rate constant of the C-O bond cleavage of these new alkoxyamines has been measured. In addition, the thermal decomposition of the alkoxyamines has been studied. Furthermore, EPR data of the corresponding new nitroxides are presented.