Identification of the paramagnetic species present in the Cu(I)Br-catalyzed atom transfer radical polymerization (ATRP) of a model monomer (isobornyl acrylate) has been carried out by electron paramagnetic resonance (EPR) in the continuous wave mode at 90 K. Up to five different species-four copper-based species and one organic radical-were detected with this technique. The EPR parameters of the copper-based species are found to differ strongly, and originate from diverse isolated Cu(II) complexes, as well as dipolarly interacting and even exchange-coupled Cu(II) species. The work highlights the complexity of the copper-based EPR signal observed in copper-mediated ATRP reactions. Analysis of the time evolution of the individual EPR contributions reveals the disadvantages of quantitative kinetics studies based on the summed EPR intensity of all copper-based species, as is commonly used in literature for this type of reactions. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1493-1501, 2010