Optimised matrix-assisted laser desorption/ionisation (MALDI) and electrospray ionisation (ESI) mass spectrometry (MIS) methodologies were systematically compared in terms of their relative abilities to identify distinct chemical species present in samples associated with a polymer mechanistic study. In order to perform the investigation, formation processes involved in atom transfer radical polymerisation (ATRP) mediated methyl acrylate (MA) star polymerisations were studied. In addition to the 4-armed ATRP initiator employed in the polymerisations, initiator side-products were found to generate oligomeric chains. At a relatively high monomer to polymer conversion, terminal Br loss was observed in these oligomers; this Br loss was hypothesised to occur via degradative transfer reactions involving the radicals (CH3)(2)(C) over dotOH, (C) over dotH(3) and (C) over dotH(2)COCH(3), which were derived from the acetone used as a solvent in the polymerisations, as well as hydrogen radicals donated by the ligand N,N',N',N '',N ''-pentamethyldiethylenetriamine (PMDETA). In performing these studies, ESI was found to identify a greater number of distinct chemical species in the samples under investigation when compared to the employed MALDI technique, suggesting that the utilisation of ESI must be strongly considered in polymer mechanistic investigations if the maximum number of end-group functionalities within a given polymer sample are to be identified. (C) 2009 Elsevier Ltd. All rights reserved.