As(III) and As(V) biosorption using Staphylococus xylosus biomass pretreated with Fe(III) solutions were investigated here. Biomass at initial concentration of 7.0 g.l(-1) was treated with 900 mg.l(-1) of Fe(III) at pH 3.0, and contact time 1.0 h. Optimum values including pH, biomass concentration and biomass-arsenic contact time, were first investigated and determined at 7.0, 1,0 g.l(-1) and 30 min for As(III) and 3.0, 2.0 g.l(-1) and 2.5 h for As(V) respectively. Potentiometric titration of the biomass and FT-IR studies showed that carboxyl groups are mainly responsible for Fe(III) binding, whereas As(III) and As(V) are adsorbed on the biomass surface through interaction with >FeOH and >FeOH2 groups. The maximum biosorption capacity was calculated using Langmuir model and found to be 54.35 and 61.34 mg.g(-1) for As(III) and As(V) respectively. Adsorbed As(III) and As(V) was fully regenerated with 0.09 M HCl at S/L equal to 2.0 showing that Fe(III)-treated biomass can be used effectively as a biosorbent for both forms of arsenic and it can be used for three subsequent adsorption/desorption cycles. (C) 2010 Elsevier B.V. All rights reserved.