Fe-carbides have been identified as the active phase in Fe-based Fischer-Tropsch (FT) catalysts. The presence of other phases than Fe-carbide, such as metallic Fe or Fe-oxide, lowers the FT activity and, especially Fe-oxides are problematic as they also catalyze the formation of CO2 from CO, implying loss of valuable C atoms. Moreover, the presence of other phases complicates the study of fundamental structure-performance relationships. Therefore, it is highly desirable to synthesize phase-pure Fe-carbide catalysts [2,3]. Here,we prepared phase-pure χ-Fe5C2 by a dedicated carburization procedure of a high-purity Fe-oxide precursor obtained by flame spray pyrolysis (FSP). The resulting phase-pure χ-Fe5C2 catalyst outperforms reference catalysts with a high CTY and a very low selectivity to undesired CO2 and CH4 [2–11]. our goal is to determine in detail the evolution of the catalyst during carburization and the FT reaction employing operando WAXS combined with MS.