Effects of pretreatment procedures, using H-2, CO, and syngas (H-2/CO = 0.7) as reductants, on the performance (activity, selectivity, and stability with time) of a precipitated iron catalyst (100 Fe/5 Cu/4.2 K/16 SiO2 on a mass basis) during Fischer-Tropsch (FT) synthesis were studied in a stirred tank slurry reactor. The syngas conversion varied between 70 and 87% during the first 300 h on stream at baseline process conditions (260 degrees C, 1.48 MPa, 1.5 L(STP)/g-cat/h, H-2/CO = 0.67), and the catalyst deactivation rate was less than 1% per day in all three tests, The catalyst selectivity did not vary markedly with time on stream. The reactor space-time-yield increased by 32-47% in all three tests, upon increasing the reaction pressure and gas space velocity to 2.17 MPa and 2.2 L(STP)/g-cat/h, respectively. The catalyst deactivation rate increased and the water-gas-shift (WGS) activity decreased during testing at the higher reaction pressure. This resulted in a slight decrease of secondary olefin hydrogenation and isomerization reactions in all three tests, and in a shift of hydrocarbon distribution toward higher molecular weight products (CO pretreated catalyst). Total olefin content decreased in the following order with the pretreatment conditions : CO > syngas > H-2, whereas the olefin isomerization activity was essentially independent of the pretreatment conditions employed, Methane selectivity varied with the pretreatment conditions in the following order : CO < H-2 < syngas.