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We measured. diphenylarsinic acid (DPAA) in well water and seawater by solid-phase (SP) extraction, followed by graphite-furnace atomic-absorption spectrometry (GFAA). The sample solution was loaded on an Oasis (R) HLB cartridge activated by 5 ml of ethanol and 20 ml of distilled water. After washing with 15 ml of distilled water, DPAA was eluted with 6 ml of ethanol. The solvent was evaporated and the residue was dissolved in distilled water. This final solution (20 mu l) was injected with a 0.025% magnesium solution (20 mu l) into GFAA, and the DPAA concentration was determined. Other arsenic compounds, positive and negative ions and humic acid were also loaded to SP. When less than I mu g of phenylarsonic acid was loaded, it was completely washed out before elution. Iron (111) reduced the recovery of DPAA, but it was improved by adding EDTA to the sample solution. The recoveries of DPAA in well water and seawater were 97% (C.V. = 2.3%, n = 5) and 102.7% (C.V. = 3.4%, n = 5), respectively. The detection limits of DPAA in well water and seawater were 0.028 ng/ml (0.008 ng/ml as arsenic) and 0.175 ng/ml (0.05 ng/ml as arsenic), respectively.