Display omitted •Carburization Raney Fe investigated by in situ and temperature-programmed methods.•CO dissociation on metallic Fe proceeds at sub-ambient temperatures.•H2 accelerates O removal: O removal via H2O is intrinsically faster than via CO2.•A higher H2/CO ratio leads to a higher C coverage and a higher carburization rate.•Fe-carbide phase formation strongly related to kinetic driving force (C coverage) The formation of Fe-carbide phases is relevant to the synthesis of Fischer-Tropsch synthesis catalysts. We investigated the carburization of Raney Fe as a model catalyst using spectroscopic and temperature-programmed techniques. IR spectroscopy shows that CO dissociation already occurs at −150 °C, while C diffusion into metallic Fe requires much higher temperature (~180 °C). The carburization rate increases with increasing H2/CO ratio, which can be attributed to the lower overall barrier for O removal as H2O as compared to CO2. O removal frees vacancies that are needed for CO dissociation. The resulting higher C coverage increases the driving force for Fe-carbide formation. A higher driving force leads to predominant formation of the more carbon-rich ε(́)-carbide, while χ-Fe5C2 is formed at lower H2/CO ratio. The removal of surface O appears to be the rate-limiting step under all conditions. Initially, most of deposited C is used for Fe-carbide formation with a small contribution to hydrocarbons formation.