Temperature-programmed reduction using both carbon monoxide (CO-TPR) and hydrogen (H-2-TPR) was used to study the phase transformations in iron catalysts. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) allowed us to follow the phase transformations in these iron catalysts during TPR. Two catalysts were used, a model supported catalyst with a nonporous silica support and a precipitated catalyst with no support. Under identical CO-TPR conditions, the supported and unsupported iron catalysts behaved very differently. For the supported catalyst, three stages of phase transformation could be identified: hematite to magnetite, magnetite to iron carbide, and in the third stage significant carbon deposition accompanying further carburization. No carbide formation or significant carbon deposition was observed in the unsupported Fe catalyst, due to the presence of S impurities in the unsupported catalyst. Most importantly, the results show that carbon deposition occurs in parallel with carbide formation, no carbon being seen in the catalyst that was not carbided. In these catalysts, copper facilitates the reduction of iron oxide, especially the transformation of hematite to magnetite. Segregation of copper from iron oxide causes the hematite to magnetite transformation to shift to higher temperatures. No interfacial Fe-SiO2 phases could be detected during both CO-TPR and H-2-TPR. The major role of the silica support is to prevent the sintering of the Fe phases.