Effects of reduction pressure in syngas (H-2/CO = 1.2), on the textural properties and bulk/surface phase compositions of a precipitated potassium promoted iron-manganese catalyst, were investigated by N-2 physisorption, X-ray diffraction (XRD), Mossbauer effect spectroscopy (MES), temperature-programmed hydrogenation (TPH) and X-ray photoelectron spectroscopy (XPS). Fischer-Tropsch synthesis (FTS) was performed in a slurry-phase continuously stirred tank reactor (STSR). The characterization results indicated that the increase of reduction pressure led to the decrease in surface area and increase in pore diameter. Pretreatment at higher reduction pressure promoted the reduction of alpha-Fe2O3 to Fe3O4 and enhanced the Boudouard reaction, whereas suppressed the carburization of magnetite. The increase of carbonaceous species concentration with increasing reduction pressure resulted in the decrease in amounts of magnetite and carbide phases in the near-surface region. In the FTS reaction, the catalytic activity of the catalyst decreased gradually and the product distribution shifted towards lower molecular weight hydrocarbons with the increase in reduction pressure. In addition, the selectivity to olefins decreased and the selectivity to oxygenates increased with increasing reduction pressure. (C) 2008 Elsevier B.V. All rights reserved.