Arsenic is considered a highly toxic metalloid, but the toxicity depends on its speciation. Inorganic species are considered more toxic than organic ones and the As(III) more toxic than As(V). This study evaluated the speciation of soluble arsenic after treatment of contaminated water by co-precipitation of Fe and Al (hydr)oxides. The results showed that the valence of Fe and presence of Al play an important role on As speciation in soluble phases resulting from water treatment.