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We report the synthesis of a new range of iron oxide–graphene oxide (GO) nanocomposites having different iron oxide content (36–80 wt%) as high-performance adsorbents for arsenic removal. Synthesized by co-precipitation of iron oxide on GO sheets that are prepared by an improved Hummers method, the iron oxide in the nanocomposites is featured primarily in the desirable form of amorphous nanoparticles with an average size of ca. 5 nm. This unique amorphous nanoparticle morphology of the iron oxide beneficially endows the nanocomposites with high surface area (up to 341 m2 g?1 for FeOx–GO-80 having the iron oxide content of 80 wt%) and predominant mesopore structures, and consequently increased adsorption sites and enhanced arsenic adsorption capacity. FeOx–GO-80 shows high maximum arsenic adsorption capacity (qmax) of 147 and 113 mg g?1 for As(III) and As(V), respectively. These values are the highest among all the iron oxide–GO/reduced GO composite adsorbents reported to date and are also comparable to the best values achieved with various sophisticatedly synthesized iron oxide nanostructures. More strikingly, FeOx–GO-80 is also demonstrated to nearly completely (>99.98%) removes arsenic by reducing the concentration from 118 (for As(III)) or 108 (for As(V)) to <0.02 μg L?1, which is far below the limit of 10 μg L?1 recommended by the World Health Organization (WHO) for drinking water. The excellent adsorption performance, along with their low cost and convenient synthesis, makes this range of adsorbents highly promising for commercial applications in drinking water purification and wastewater treatment.