The seleno-bis (S-glutathionyl) arsinium ion, [(GS) 2 AsSe] -, which can be synthesized from arsenite, selenite and glutathione (GSH) at physiological pH, fundamentally links the mammalian metabolism of arsenite with that of selenite and is potentially involved in the chronic toxicity/carcinogenicity of inorganic arsenic. A mammalian metabolite of inorganic arsenic, dimethylarsinic acid, reacts with selenite and GSH in a similar manner to form the dimethyldiselenoarsinate anion, [(CH 3 ) 2 As(Se) 2 ] -. Since dimethylarsinic acid is an environmentally abundant arsenic compound that could interfere with the mammalian metabolism of the essential trace element selenium via the in vivo formation of [(CH 3 ) 2 As(Se) 2 ] -, a chromatographic method was developed to rapidly identify this compound in aqueous samples. Using an inductively coupled plasma atomic emission spectrometer (ICP-AES) as the simultaneous arsenic-and selenium-specific detector, the chromatographic retention behaviour of [(CH 3 ) 2 As(Se) 2 ] -was investigated on styrene-divinylbenzene-based high-performance liquid chromatography (HPLC) columns. With a Hamilton PRP-1 column as the stationary phase (250 × 4.1 mm ID, equipped with a guard column) and a phosphate-buffered saline buffer (0.01 mol dm -3 , pH 7.4) as the mobile phase, [(CH 3 ) 2 As(Se) 2 ] -was identified in the column effluent according to its arsenic : selenium molar ratio of 1 : 2. With this stationary phase/mobile phase combination, [(CH 3 ) 2 As(Se) 2 ] -was baseline-separated from arsenite, selenite, dimethylarsinate, methylarsonate and low molecular weight thiols (GSH, oxidized GSH) that are frequently encountered in biological samples. Thus, the HPLC-ICP-AES method developed should be useful for rapid identification and quantification of [(CH 3 ) 2 As(Se) 2 ] -in biological fluids.