NMR spectroscopy confirmed the existence of a C-Br bond at the terminal chain end of low molecular weight poly(methyl methacrylate) (PMMA) produced by atom transfer radical polymerisation (ATRP). Polymer was prepared using the heterogeneous Cu(I)Br/2,2'-bipyridine (bpy) catalyst with ethyl-2-bromoisobutyrate used as the initiator. This structural feature was not evident in the matrix-assisted laser desorption/ionisation-time of flight (MALDI-TOF) mass spectrometry spectrum of the same polymer sample. The mass-to-charge ratios of the molecule ion peaks corresponded to a loss of methyl bromide (MeBr) from one chain end and concomitant cyclisation to give a lactone end group. Further information was obtained by means of matrix-assisted laser desorption/ionisation-collision induced dissociation (MALDI-CID) mass spectrometry. These spectra showed fragmentation patterns that were consistent with that shown previously for other alkyl methacrylate polymers. Evolved gas analysis detected the presence of MeBr in the volatiles collected when the polymer was heated from ambient temperature to 150 degrees C. Furthermore, samples of PMMA, which had been previously heated to 150 degrees C under an atmosphere of dinitrogen, were analysed by C-13 NMR. Spectra were similar to the samples that had not been heated apart from a complete loss of the C-Br resonance at delta 58 ppm and the appearance of other signals consistent with lactone formation. This was taken as evidence that PMMA terminated by a Br atom in this way, will undergo a thermal decomposition at similar to 150 degrees C that is similar to that observed in the MALDI experiments.