An automated on-line pre-reduction of arsenate, monomethylarsonate (MMA) and dimethylarsinate (DMA) using flow injection hydride generation atomic absorption spectrometry (FI-HGAAS) is feasible. The kinetics of pre-reduction and complexation depend strongly on the concentration of l-cysteine and on the temperature in the following increasing order: inorganic As(V)<DMA<MMA. Arsenate is pre-reduced/complexed within less than 50 s at 70-100 o C compared to 1 h at room temperature, while MMA and DMA require 1.5-2 min at 70-100 o C and up to 1-2 h at room temperature. The characteristic masses and concentrations for 100 μl injections are 0.01 ng and 0.1 μg l - 1 in integrated absorbance and 0.2 ng and 2 μg l - 1 in peak height measurements, and the limits of detection are ca. 0.5 ng and 5 μg l - 1 , respectively. In a high-performance liquid chromatography (HPLC)-HGAAS system, the l-cysteine complexes of inorganic As(III), MMA and DMA are best separated within 7 min by HPLC on a strongly acidic cation exchange column such as Spherisorb S SCX 120x4 mm (5 μm) with a mobile phase containing 12 mmol l - 1 phosphate buffer (KH 2 PO 4 /H 3 PO 4 )-2.5 mmol l - 1 l-cysteine, pH 3.3-3.5. Upon dilution to l-cysteine levels below 10 mmol l - 1 , which are compatible with HPLC separations, the DMA-cysteine complex is unstable on storage. No baseline separations are possible with anion exchange and reverse phase C 1 8 HPLC columns. The limits of detection with 50 μl injections in peak area mode are ca. 0.5 ng and 10 μg l - 1 , respectively.