Intrinsic rates for the Fischer-Tropsch synthesis reaction over a promoted iron catalyst fabricated at the Research Institute of the Petroleum Industry (RIPI) have been obtained in the temperature range of 290 degreesC to 310 degreesC, pressure range of 1500 to 2300 kPa, molar hydrogen to carbon monoxide ratio of 0.76 to 1.82, and a space velocity of 3300 h(-1) under conditions of constant catalyst activity. To this end, the initial reaction rates have been measured at constant temperature (+/-1 degreesC) in the absence of diffusion limitations, and power-law equations have been fitted in terms of the hydrogen and carbon monoxide partial pressures for the reaction rates.