Diphenylarsinic acid (DPAA) is a toxic phenylarsenical compound often found around sites contaminated with phenylarsenic chemical warfare agents, diphenylcyanoarsine or diphenylchloroarsine, which were buried in soil after the World Wars. This research concerns the elucidation of the chemical structure of an arsenic metabolite transformed from DPAA under anaerobic sulfate-reducing soil conditions. In LC/ICP-MS analysis, the retention time of the metabolite was identical to that of a major phenylarsenical compound synthesized by chemical reaction of DPAA and hydrogen sulfide. Moreover the mass spectra for the two compounds measured using LC/TOF-MS were similar. Subsequent high resolution mass spectral analysis indicated that two major ions at m/z 261 and 279, observed on both mass spectra, were attributable to C12H10AsS and C12H12AsSO, respectively. These findings strongly suggest that the latter ion is the molecular-related ion ([M+H](+)) of diphenylthioarsinic acid (DPTA; (C6H5)(2)AsS(OH)) and the former ion is its dehydrated fragment. Thus, our results reveal that DPAA can be transformed to DPTA, as a major metabolite, under sulfate-reducing soil conditions. Moreover, formation of diphenyldithioarsinic acid and subsequent dimerization were predicted by the chemical reaction analysis of DPAA with hydrogen sulfide. This is the first report to elucidate the occurrence of DPAA-thionation in an anaerobic soil. (C) 2013 Elsevier B.V. All rights reserved.