It is vital that methylated trivalent arsenicals [MA(III) and DMA(III)] are described and characterized unequivocally due to their high toxicity. Two different ways of generating the methylated trivalent arsenicals have been practiced-reduction of the methylated pentavalent arsenical either by the sodium-metabisulfite (Na(2)S(2)O(5))/sodium thiosulfate (Na(2)S(2)O(3)) reagent (method A) or by KI, H(2)SO(4), and SO(2) (method B). The shared identity between the products of the two synthetic methods has never been questioned or proven. Here, we characterize and identify the arsenic species formed when reducing DMA(V) by method A or B. Dimethylarsinous acid [DMA(III)] was formed when reducing DMA(V) by method B, but DMA(III) was not the main product of the reaction by method A. The product was revealed by HPLC-ICP-MS coupled simultaneously to HPLC-ES-MS and ES-Q-TOF-MS to have the molecular formula C(2)H(7)OSAs. The structure was further confirmed by (1)H NMR, and ab initio tautomeric energy calculations showed it to be present as Me(2)As(=S)OH (dimethylarsinothioic acid). Dimethylarsinothioic acid was also identified as a metabolite in urine and in wool extract from sheep naturally consuming large amounts of arsenosugars (35 mg of As daily) through their major food source, seaweed.