A method using bi-directional electrostacking (BDES) in a flow system is presented for As preconcentration and speciation analysis. Some parameters such as electrostacking time and applied voltage, support buffers and their concentrations were investigated. Boric acid plus sodium hydroxide at 0.1 mol/l concentration was selected as support buffer to improve the pre-concentration factor (PF) for As(V). An analytical range from 2.0 to 50.0 mu g l(-1). and 0.35 mu g l(-1) as limit of detection, when applied 750 V for 20 min, were achieved. Under these conditions, a pre-concentration factor of 4.8 was obtained. The proposed method was applied to determine As(V) in mineral water and natural water samples (river, fountain and gold mine) from Ouro Preto city. Recoveries from 93.5 to 106.4% were achieved at 10 mu g l(-1) added As level (R.S.D.s between 3 and 7%). Potassium permanganate (10 mg l(-1)) was used for oxidising As species in order to determine total As, being established the concentration of As(III) from the difference between total As and As(V). (c) 2006 Elsevier B.V. All rights reserved.711353358