We report a new synthesis of the seleno-bis (S-glutathionyl) arsinium anion, [(GS)2AsSe]−. An aqueous solution of bis-glutathionylarsenous acid, (GS)2AsOH, prepared from stoichiometric glutathione and arsenite, was reacted in situ with a solution of sodium hydrogen selenide, prepared from elemental selenium and sodium borohydride. Analysis of the arsenic and selenium K-edge X-ray absorption spectra indicated virtually quantitative formation of [(GS)2AsSe]−, with AsSe and AsS distances of 2.31 and 2.25 Å, respectively, and the concentrated sample allowed a definitive X-ray spectroscopic characterization. Size-exclusion chromatography was used to separate [(GS)2AsSe]− from residual borate in the reaction mixture.