The traditional techniques for removing low concentration arsenic are unsuitable. The biosorption characteristics of arsenite (iAs(III)) and monomethyl arsonate (MMA(V)) from aqueous solution by Acidithiobacillus ferrooxidans BY-3 (At. f BY-3) were investigated as a function of pH, contact time, initial arsenic concentration, biomass dosage and temperature in this study. Results indicated that Langmuir isotherm model fitted better than Freundlich model to the equilibrium data. Analysis of kinetic data showed that the biosorption processes of both iAs(III) and MMA(V) involved pseudo-second-order kinetics. The thermodynamic parameters such as DeltaG(o), DeltaH(o) and DeltaS(o) of the biosorption process showed that the adsorption of iAs(III) and MMA(V) onto At. f BY-3 was feasible, spontaneous and endothermic under the examined conditions. The competitive biosorption of iAs(III) and MMA(V) in binary mixture system was evaluated, and the results indicated that At. f BY-3 favored MMA(V) biosorption. Fourier-transform infrared spectroscopy (FT-IR) showed -OH and -NH groups were involved in the biosorption process.