The reactive growth of ultrathin Fe oxide films on Ru(0001) has been studied and characterized using low-energy electron microscopy, diffraction, and laterally resolved spectroscopies. Under exposure to molecular oxygen at 900 K, we observed the growth of a bicomponent film composed of micrometer-sized flat triangular Fe3O4 (magnetite) islands on a FeO (wüstite) wetting layer. Subsequent oxidation using NO2 at 600 K resulted in the chemical transformation of the initially grown film to a Fe2O3 composition but still in bicomponent form. The triangular magnetite islands evolve to γ-Fe2O3 (maghemite), and the surrounding layer is converted to α-Fe2O3 (hematite). The evolution of both members of the bicomponent iron oxide films, wüstite to hematite and magnetite to maghemite, can be understood by considering that both are topotactic transformations occurring by the diffusion of iron in octahedral sites to react with oxygen on the film’s surface.