It has been suggested recently that arsenic-glutathione (As-GSH) complexes play an important role in the methylation of arsenic. The present study describes the development of high-performance liquid chromatography (HPLC)-electrospray tandem mass spectrometry (ES-MS/MS), operated in the selected reaction monitoring (SRM) mode, and HPLC-inductively coupled plasma mass spectrometry (ICP-MS) methods suitable for the sensitive and selective identification of four As-GSH complexes. Method optimization was carried out using a series of synthetically prepared standards, i.e., three As-GSH species containing trivalent arsenic: tri(glutamyl-cysteinyl-glycinyl)trithio-arsenite (ATG), di(glutamyl-cysteinyl-glycinyl)methyl-dithio-arsonite (MADG), and (ã-glutamyl-cysteinyl-glycinyl) dimethyl-thio-arsinite (DMAG), as well as one As-GSH species containing pentavalent As: dimethylthioarsinic acid-glutathione (DMTA(V)-GSH). The collision induced dissociation behavior of these compounds was investigated in detail to identify optimum SRM transitions for each complex. Both methods were based on reversed-phase chromatography using gradient elution with methanol, formic acid, and water as solvents. The amount of methanol that was used with this HPLC method (up to 12% vol/vol) was compatible with ICP-MS, without the need of a specially adapted interface. Subsequently, these analytical methods were applied to carry out a preliminary investigation about the role of As-GSH complexes in the methylation of arsenite by methylcobalamin (CH(3)B(12)) in the presence of glutathione (GSH). For the first time, the complexes ATG, MADG, and trace amounts of DMAG were detected as products of this reaction.