In most natural groundwaters, sulfide concentrations are low and little attention has been paid to potential occurrence of thioarsenates (AsVSn-IIO4-n3- with n=1-4). Thioarsenate occurrence in groundwater could be critical with regard to the efficiency of iron (Fe)-based treatment technologies because previous studies reported less sorption of thioarsenates to preformed Fe-minerals compared to arsenite and arsenate. We analyzed 273 groundwater samples taken from different wells in Bangladesh over a year and detected monothioarsenate (MTA), likely formed via solid-phase zero-valent sulfur, in almost 50% of all samples. Concentrations ranged up to >30 µg L-1 (21% of total As). MTA removal by locally used technologies in which zero-valent or ferrous Fe is oxidized by aeration and As sorbs or co-precipitates with the forming Fe(III)hydroxides was indeed lower than for arsenate. The presence of phosphate required up to three times as much Fe(II) for comparable MTA removal. However, in contrast to previous sorption studies on preformed Fe minerals, MTA removal, even in the presence of phosphate, was still higher than that of arsenite. The more efficient MTA removal is likely caused by a combination of co-precipitation and adsorption rendering the tested Fe-based treatment technologies suitable for As removal also in the presence of MTA.