Magnetite ( Fe3O4) was synthesized via the hydrothermal method and then used as an adsorbent for Cr6+ removal from water. Cr-adsorbed Fe3O4 (Cr/Fe3O4) with various Cr concentrations were then applied as catalysts for glucose dehydration in aqueous solution. The role of Cr-adsorbed species for the catalytic performance was investigated. The absorption ability and reducibility of the Fe3O4 for Cr6+ were investigated. The maximum adsorption ability of 3.3 mg/gFe3O4 was determined at 10 ppm Cr6+ concentration using UV-Visible spectroscopy. The Fe and Cr species on Cr/Fe3O4 investigated by X-ray absorption spectroscopy were in trivalent forms of FeOOH and CrOOH which clearly resulted in a reduction of Cr6+ on the Fe3O4 surface. The existent structures corresponded to the BrØnsted acidic sites. This result clearly confirmed the reduction ability of Fe3O4 to transform Cr6+ to Cr3+ . The catalytic performances of the Fe3O4 and Cr/ Fe3O4 acidic catalysts were investigated for glucose dehydration in an aqueous medium under hydrothermal conditions. The presence of the FeOOH and CrOOH species on the Cr/Fe3O4 catalyst clearly improved their catalytic performance for glucose dehydration to a 5- hydroxymehylfurfural (HMF) product. About 50% of the HMF yield was reached by 20 ppm Cr/ Fe3O4. The maximum turnover number was observed by 1 ppm of Cr/Fe3O4 catalyst. The improvement of the HMF yield was strongly dependent on the Cr-adsorbed species and dispersion on the Fe3O4 surface. [ABSTRACT FROM AUTHOR]