A systematic study was undertaken to investigate the effects of the initial oxidation degree of iron on the bulk phase composition and reduction/carburization behaviors of a Fe-Mn-K/SiO2 catalyst prepared from ferrous sulfate. The catalyst samples were characterized by powder X-ray diffraction (XRD), Mossbauer spectroscopy, X-ray photoelectron spectroscopy (XP5) and H-2 (or CO) temperature-programmed reduction (TPR). The Fischer-Tropsch synthesis (FTS) performance of the catalysts was studied in a slurry-phase continuously stirred tank reactor (CSTR). The characterization results indicated that the fresh catalysts are mainly composed of alpha-Fe2O3 and Fe3O4, and the crystallite size of iron oxides is decreased with the increase of the initial oxidation degree of iron. The catalyst with high content of alpha-Fe2O3 in its as-prepared state has high content of iron carbides after being reduced in syngas. However, the catalyst with high content of Fe3O4 in its as-prepared state cannot be easily carburized in CO and syngas. FTS reaction study indicates that Fe-05 (Fe3+/Fe-total = 1.0) has the highest CO conversion, whereas Fe-03 (Fe3+/Fe-total = 0.55) has the lowest activity. The catalyst with high CO conversion has a high selectivity to gaseous hydrocarbons (C-1-C-4) and low selectivity to heavy hydrocarbons (C5+). (C) 2009 Elsevier B.V. All rights reserved.