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A method for the determination of diphenylarsinic acid (DPAA) in plants and biological samples by high-performance liquid chromatography-tandem mass spectrometry (HPLC/MS/MS) using a stable isotope labeled compound as an internal standard is described. After the sample was decomposed in 2 M sodium hydroxide by heating, an internal standard (DPAA-C-13(12)) was added to the solution. The decomposed solution was shaked with a mixture of chloroform and n-hexane (1 : 4). After centrifugal separation, the organic layer was discarded. The aqueous layer was then adjusted to ca. 0.5 M hydrochloric acid solution, and cysteine and potassium iodide were added in order to extract DPAA efficiently in the chloroform. Extraction was carried out twice, and the chloroform layers were combined and evaporated. The extracts were dissolved in methanol or water in case of plants. When a colored solution was obtained, it was purified by an Oasis HLB solid-phase cartridge. In biological materials, the extracts were dissolved in a small volume of methanol, and then 0.5% nitric acid solution was added. When the mixture got cloudy, it was passed through a 0.20 mu m of membranfilter. DPAA in the final solution was analyzed by HPLC/MS/MS. HPLC separation was performed with an ODS column using a linear gradient elution of an acetonitrile-water system containing 0.1% trifluoroacetic acid as the mobile phase. MS/MS was used in multiple reaction monitoring (MRM), employing electrospray ionization. The proposed method could determine DPAA in plants and biological samples with an average recovery of 99.4%. The coefficient of variation was as low as 6%. The calibration curve was linear between 0 and 30 ng/ml.