Hexavalent chromium represents higher toxicity in aqueous solutions. It can be removed by such low-cost biosorbents as Sargassum sp. In this study, X-ray absorption fine structure spectroscopy, X-ray photoelectron spectroscopy, and quantum chemistry (QC) calculation were used to study the interactions between hexavalent chromium and Sargassum sp. during the biosorption. It was found that most of the adsorbed Cr(VI) ions were reduced to Cr(III) after the biosorption. The electrons for the reduction of Cr(VI) were possibly supplied from the Sargassum biomass, some organic compounds of which were oxidized. Cr(III) ions were coordinated with the oxygen atoms from either carboxyl or hydroxyl functional groups to form an octahedral structural metal complex. The coordination numbers of the formed Cr complex were 4-6, and bond length of Cr-O was 1.98 angstrom. QC calculation proved the possible formation of the Cr(III) metal complex, and revealed that carboxyl from biomass could be strongly bound with Cr(III). A three-step removal mechanism of Cr(VI) by Sargassum was proposed. (C) 2010 Elsevier Inc. All rights reserved.