The effect of potassium on oxides and carbides of iron for Fischer-Tropsch synthesis (FTS) was investigated by pretreating Fe3O4 and K-promoted Fe catalysts with different gases (H-2/H2O and CO). A freshly activated sample and catalysts that were recovered from the CSTR before, during and after FT synthesis were characterized ex situ using Mossbauer spectroscopy. Iron carbide is found to be active for both FT and water gas shift (WGS) reactions. After H-2/H2O activation, all three catalysts (Fe3O4, low alpha-Fe, and high alpha-Fe) exhibit a steady but low FT activity for a period of FT synthesis. However, both FT and WGS activity for Fe3O4 and low alpha-Fe catalysts were greatly improved after CO activation. In contrast, the high potassium containing catalyst (high alpha-Fe) did not show any further improvement in activity after CO activation. The difference in FT and WGS activity observed after pretreatment conditions using these catalysts may be associated to the amount of potassium and conversely the iron carbide present in the catalysts.