This study investigated arsenate and arsenite adsorption on an iron(III) oxide/silica adsorbent that had a Fe/Si molar ratio of 3. Batch experiments were conducted with regard to adsorption kinetics, isotherms, pH effects and interference of phosphate and sulfate on arsenic adsorption. The adsorption of arsenite was faster than that of arsenate. The adsorption kinetics could be well described by the power function equation for both arsenate and arsenite. The arsenate and arsenite adsorption isotherm data were satisfactorily fitted by the Redlich-Peterson, Freundlich-Langmuir and Langmuir models. The equilibrium adsorption capacity for As(III) was larger than that for As(V). This capacity for both As(III) and As(V) increased with temperature, from 11.3 to 14.9 (mg As/g) for As(V) and from 21.1 to 21.5 (mg As/g) for As(III) when the temperature was increased from 20 to 35°C. The arsenate adsorption was significantly reduced at pH above 7.5, while the arsenite adsorption was almost independent of pH in the range of 3 to 9. The calculated thermodynamic parameters including ΔG°, ΔH° and ΔS° reflected the spontaneous nature of As(V) and As(III) adsorption on this binary oxide adsorbent. The opposite values of ΔH° indicated that the adsorption of As(V) was exothermic, whereas that of As(III) was endothermic. It was found that the interference of phosphate at pH 7.4 was more significant for arsenate adsorption than for arsenite adsorption. In contrast, sulfate had insignificant interference on arsenic adsorption at the tested sulfate concentration up to 460 mg/L and pH 7.4.