Organoarsenic compounds have been widely used as feed additives in livestock and poultry breeding. They are excreted in manure and thus cause environmental pollution. Moreover, organoarsenic compounds can be transformed into other arsenic species with higher toxicity. There is little knowledge on the transformation of organoarsenic compounds in the presence of iron species. Here, we studied the transformation of roxarsone (4-hydroxy-3-nitrophenylarsonic acid) in the presence of ferrous ion (Fe(II)) and tetrapolyphosphate. Various generated arsenic species were monitored. Results show that 94.8% of roxarsone was degraded within 30 min in 10 mmol L−1 Fe(II) and 50 mmol L−1 tetrapolyphosphate under air atmosphere. 70% of roxarsone arsenic was converted to As(V) species by hydroxyl radicals generated in the air system. Moreover, roxarsone is reductively transformed into other organoarsenic compounds, for example 4-hydroxy-3-aminophenylarsonic acid, by accepting electrons from the Fe(II)-tetrapolyphosphate complex. The toxicity and mobility of the generated inorganic or organic arsenic species are higher than those of roxarsone, which may induce serious environment and human health problems.