Slurry phase Fischer-Tropsch synthesis (FTS) was conducted with two precipitated iron catalysts (100 Fe/3.6 Si/0.71 K and 100 Fe/4.4 Si/1.0 K, atomic percent relative to Fe) at 543 K, 1.31 MPa, and a synthesis gas (H-2/CO = 0.7) space velocity of 3.1 normal L h(-1) (g of Fe)(-1). The impact of activation gas (GO, H-2/CO = 0.7, or H-2/CO = 0.1), temperature (543 or 573 K), and pressure (1.31 or 0.10 MPa) on the long-term (>500 h) activity and selectivity of the catalysts was explored. Pretreatment with CO under the conditions employed gave highly active and stable catalysts. Catalyst performance when synthesis gas activation was used was found to be dependent upon the partial pressure of hydrogen in the activating gas, with low hydrogen partial pressures resulting in the highest catalyst activity. X-ray diffraction results indicate that carbon monoxide activations and synthesis gas activations with low hydrogen partial pressure result in the formation of the carbides chi-Fe5C2 and epsilon’-Fe2.2C, while activation with synthesis gas with high hydrogen partial pressure results in the formation of only Fe3O4. It was found that treating the 100 Fe/3.6 Si/0.71 K catalyst activated with synthesis gas at 1.31 MPa and 543 K with carbon monoxide caused the activity to increase dramatically and the Fe3O4 to be partially converted to iron carbides. It is concluded that Fe3O4 is relatively inactive for FTS, while the presence of some bulk iron carbide is necessary for high FTS activity to be achieved.