Peatlands efficiently sequester inorganic arsenic as arsenite or arsenate by binding to natural organic matter via O-containing functional groups or S- and Fe-bridging. Microbial activity can lead to the formation of organic arsenates that can successively form organic thioarsenates in the presence of sulfide. While low sorption of inorganic thioarsenates has been shown before, until now, the sorption affinity of organic (thio)arsenates to peat functional groups and the mobility in peat porewater are poorly understood. Here, we show that substantial fractions of porewater arsenic from the first 15 cm of a natural, minerotrophic, slightly acidic riparian peatland “Schlöppnerbrunnen II” were organic thioarsenates (60%) with dimethylmonothioarsenate (DMMTA) being the dominant arsenic species. Depth-resolved porewater analysis showed that inorganic and organic thioarsenates only occurred in high fractions when redox conditions got anoxic and sulfide concentrations increased. Laboratory sorption experiments with three different peats (low iron, high iron, high sulfide) confirmed mobility of organic (thio)arsenates in peatland porewater by showing no affinity of monomethylarsenate (MMA), dimethylarsenate (DMA), and DMMTA at pH 4, 5.5, and 7 to peat functional groups after 96 h. While MMA and DMA were stable, 20-50 % of DMMTA dissociated to DMA and sulfide at circumneutral pH and subsequently dimethyldithioarsenate (DMDTA) formed by reaction of DMMTA with sulfide. Our results show that organic thioarsenates are highly mobile in peatland porewater and that their high fraction has been previously overlooked in routine analyses due to insufficient sample preservation and analysis methods.