The polymerisation of styrene in the presence of cumyl dithiobenzoate (RAFT agent) was studied. Electron spin resonance spectroscopy was used to determine the time-dependence of the concentrations of the intermediate radical species, as a function of temperature and relative initial amounts of reactants. The intermediate radical concentration was found to be much lower than predicted by the studies of Barner-Kowollik et al.[Barner-Kowollik, C.; Quinn, J. F.; Morsley, D. R.; Davis, T. P. J. Polym. Sci. Part A: Polym. Chem.2001, 39, 1353], but consistent with that observed by Hawthorne et al.[Hawthorne, D. G.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules1999, 32, 5457] and Kwak et al.[Kwak, Y.; Goto, A.; Tsujii, Y.; Murata, Y.; Komatsu, K.; Fukuda, T. Macromolecules2002, 35, 3026] Several observations that are not consistent with the current RAFT model were made. Intermediate radical signals were observed long after a significant amount of initiator remained and an extra signal appeared in the ESR spectrum at longer reaction times.