In order to elucidate the activation mechanism of iron based Fischer-Tropsch catalysts, we performed in situ nanoscale structural and compositional analysis during carburization in CO atmosphere. We found that inside an ETEM at 20 Torr CO, hematite reduces while severe sintering occurs. This sintering is enhanced by low pressure conditions in combination with CO gas. The reduction inside the ETEM is so effective that metallic iron can be formed. During the process, about 50 nm sized iron carbide crystallites grow out of the iron (oxide) particles. Upon prolonged reaction, these newly formed crystallites reduce in size to about 5-30 nm. In contrast to what has been described in the literature until now, no carbon deposits were found on the outer rim of these iron carbides at high temperature conditions. Note however that, in contrast to our measurements, previous studies involved catalysts that were cooled or passivated prior to (TEM) analysis. (C) 2010 Elsevier B.V. All rights reserved.