The effects of sulfur modification (S/Fe = 0.001 similar to 0.05, molar ratio) on the alpha-Al2O3-supported iron catalysts for the Fischer-Tropsch synthesis (FTS) were investigated. It was found that the presence of sulfur promoted the reduction of FeO to metallic Fe during the H-2 reduction process, but decreased the rates of reduction, carburization and carbon deposition of the catalysts when CO is used as the reducing agent. Both the sulfide (S2-) and sulfate species (SO42-) are found in the SxFe/alpha-Al2O3 catalysts after H-2 reduction and FTS reaction. The sulfide species in the H-2-reduced SxFe/alpha-Al2O3 catalysts are located on the surface of metallic Fe particles. The intensity of H-2 desorption peak around 500 K is enhanced by a small amount of sulfur, while an excess of sulfur suppresses the H-2 adsorption. The dissociative adsorption of CO on H-2-reduced catalysts was also found to be inhibited by the presence of sulfur. The activity of the Fe/alpha-Al2O3 catalyst can be decreased significantly by the addition of sulfur due to the inhibition of CO dissociation and thus reduce the formation of iron carbide phases under FTS reaction conditions. Resistance to sulfur poisoning of the Fe/alpha-Al2O3 catalyst was found to improve with increasing the reaction temperature. The presence of sulfur also suppressed the formation of C5+ hydrocarbons and shifted the products to C-2-C-4 hydrocarbons. At the same time, the olefin to paraffin (O/P) ratio of the C-2-C-4 hydrocarbons decreased with increasing S/Fe molar ratio. These may have resulted from the increasing of the H/C ratio on the surface of sulfur modified catalyst under FTS conditions. (C) 2015 Elsevier B.V. All rights reserved.