The type of the precipitating agent used during the preparation of a precipitated iron-based Fischcr-Tropsch (FT) catalyst affects the catalyst pore structure, crystallite size, phase composition and catalytic behavior. Catalysts prepared by using precipitating agents, that contain carbonate ions, have pores that are larger than those of catalysts prepared using precipitating agents that contain hydroxides. Precipitation at pH > 8, using aqueous NH3 solution as a precipitating agent, results in the formation of large crystallites of FeOOH, which are not observed when Na2CO3 and K2CO3 are used. Higher % CO conversion during FT synthesis was observed with the catalyst prepared by using aqueous NH3 solution. However, this is correlated with a low selectivity for the formation of olefins. For all catalysts, in situ Mossbauer spectra recorded during FT synthesis show that the % CO conversion increases with the formation of iron carbides, viz. epsilon'-Fe2.2C and chi-Fe2.5C.