The thermal dissociation of the bisalkoxyamine [GRAPHICS] (denoted StNNSt) releases the styryl St(-) and one of the two isomers of the StNN(-) nitroxide radicals; the former initiates a living chain P- and the latter controls the polymerisation by limiting the chain growth. A sequence of dissociation, propagation and recombination reactions produces the StNNP and PNNP bisalkoxyamines as well as a PNN- nitroxide. A decomposition side reaction of PNNP leads to HNNP with a hydroxylamine functionality, which by dissociation produces the HNN- nitroxide. All these reactions have previously been shown to play a key role in the controlled radical polymerisation of styrene mediated by a dinitroxide biradical [1,2]. The evolution of nitroxide radicals in the course of the bulk polymerisation of styrene at 130 degreesC initiated by StNNSt, has been followed by ESR at this temperature as well as on several samples cooled down from 130 to 21 T at different time intervals. The hyperfine coupling parameters and the reorientation correlation times have been obtained from the automated fitting of spectra. As the polymerisation proceeds, the StNN(-) radical initially observed is progressively replaced by PNN-, characterised. by a much longer correlation time, which increases as the chain segmental motion slows down. At high polymer yields (60-80%) the spectra show two components S and F relevant to the slow and fast motional regimes. The S component corresponds to the fraction of PNN- embedded in the polymer chain network whereas the F component is assigned to the fraction of PNN- moving in free volumes as well as to HNN-whose concentration increases till the end of the reaction.