We used 13CO labeling to show that gas‐phase NMR spectroscopy and mass spectrometry are simple tools for mechanistic investigations of the Fischer–Tropsch (FT) reaction. Thus, monodisperse Fe nanoparticles (NPs) react with syngas to form monodisperse iron carbide (FeCx) NPs. As expected, the heating of 13C‐labeled monodisperse FeCx NPs under H2 results in the desorption of the carbide carbons as 13CH4 and, interestingly, restores the initial Fe NPs in terms of size and dispersity. The Fe13Cx NPs catalyze the hydrogenation of 12CO at 210 °C to yield only 12C‐labeled FT products, which evidences the absence of the incorporation of FeCx carbon atoms in the products. In addition, this approach shows for the first time that the formation of 13CH4 at 250 °C does not result from direct carbide hydrogenation but from an intermediate step that involves a reaction between Fe13Cx and H2O to give 13CO2, which is subsequently hydrogenated. These results rule out the involvement of FeCx carbon atoms in the chain growth process under our conditions.