The conversion of unpromoted, unsupported metallic iron catalysts into carbides during Fischer-Tropsch synthesis (CO:H2:He = 1:1:3, 1 atm) was studied with Mössbauer spectroscopy, X-ray diffraction, carbon content analysis, and reaction kinetic measurements. From a comparison between experiments at different temperatures and literature data, it is concluded that both reaction conditions and the nature of the iron catalyst determine the combination of carbides that will be formed. Investigation of single-phase carbides revealed that the X-ray diffraction pattern commonly ascribed to a pseudohexagonal carbide Fe2C actually belongs to the carbide ∈′-Fe2.2C. At synthesis temperatures of 513 K and lower, unknown iron-carbon species were found, referred to as FexC. It is believed that FexC represents poorly defined structures between α-Fe and a crystallographic carbide. The behavior of metallic iron catalysts during Fischer-Tropseh synthesis at 513 K was studied in more detail as a function of time. It was found that the rate of hydrocarbon formation was initially low, passed through a maximum, and decreased thereupon, while the conversion of α-Fe into carbides started at a high rate and decreased rapidly. These results can be understood as the consequence of either a competition between bulk carbidization and hydrocarbon synthesis or a relatively slow activation of α-Fe for the formation of hydrocarbons in which bulk carbidization plays no role. Deactivation is caused by the formation of an excessive amount of inactive carbon at the surface of the catalyst.