Chromium(VI) and arsenic(V) biosorption in single and binary mixtures using Fe(III)-treated Staphylococcus xylosus were investigated and compared. In single biosorption experiments, the maximum uptake capacity of both Cr(VI) and As(V) was observed at pH 3.0 and biomass dosage 2.0 g/L. Langmuir isotherm model described the adsorption equilibrium for both Cr(VI) and As(V) biosorption better than Freundlich isotherm model. The maximum biosorption capacity was found to be 43.48 and 61.34 mg/g of Cr(VI) and As(V) respectively. In binary mixtures, the maximum uptake capacity of dominant Cr(VI) ion was found to decrease from 41.84 to 19.08 mg/g upon addition of As(V) as competing ion from 0 to 50 mg/L, whereas that of dominant As(V) ion was decreased from 56.5 to 43.10 mg/g with the addition of competing Cr(VI) ion from 0 to 100 mg/L respectively. The results indicated that Fe(Ill)-treated S. xylosus biomass can be used effectively for the simultaneous removal of Cr(VI) and As(V) even present at high concentrations under the same conditions of pH and biomass dosage. The spontaneous and exothermic nature of Cr(VI) and As(V) sorption on Fe(III)-treated S. xylosus biomass was also indicated by the values of thermodynamic parameters calculated here. (C) 2011 Elsevier B.V. All rights reserved.