AbstractIron‐based catalysts are employed in CO₂‐FTS due to their ability to convert CO₂ into CO in a first step and their selectivity towards higher hydrocarbons in a second CO hydrogenation step. According to the literature, iron carbides represent the active phase for hydrocarbon formation and are claimed to emerge in the presence of CO. We propose nanostructured FeO_x films as model systems to assess information about the complex phase transformations during CO₂‐FTS. Mesoporous hematite, ferrihydrite, maghemite, maghemite/magnetite films were exposed to CO₂‐FTS atmospheres at 20 bar and 300 °C. Up to three distinct phases were observed depending on the time‐on‐stream (TOS): a sintered maghemite/magnetite phase, a carbidic core‐shell structure, and a low‐crystalline, needle‐type oxide phase. Our findings indicate that the formation of an intermediary maghemite/magnetite phase, predominant after short TOS (30 h), precedes the evolution of the carbide phase. Yet, even after prolonged TOS (185 h), no full conversion into a bulk carbide is observed.