During the preparation of sodium 3,5-dibromo-4-nitrosobenzenesulphonate (DBNBS) of high purity for electron paramagnetic resonance (EPR) spin-trapping purposes, it was found that the material synthesised as part of the present study differed significantly from some commercially available samples of DBNBS. A thorough chemical characterisation of the contents of the various samples led to the conclusion that the preparations synthesised in the present study, as well as one of four commercially available samples, contained essentially pure DBNBS and had efficient spin-trapping activity. In contrast, the remaining three commercially available samples contained almost exclusively sodium 3,5-dibromo-4-nitrobenzenesulphonate, i.e. a one-oxygen oxidation product of DBNBS, and had little spin-trapping activity. The two compounds were readily separated by reverse-phase high performance liquid chromatography (HPLC). It was further found that the quality of DBNBS preparations may be determined by NMR spectrometry, IR spectrometry, fast atom bombardment-mass spectrometry (FAB-MS) and EPR spectrometry. In particular, UV-Visible spectroscopy may be used to determine A308/A280, which should be greater than 1.8 for a high purity DBNBS preparation.