Heteroepitaxial growth of thin iron oxide films on Ru(0001) is investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Metastable FeO(I 11) layers grow with a thickness up to 4 monolayers (ML) on Ru(0001) by oxidation of the corresponding amount Fe in 10(-6) mbar O-2 at 870 K with a final anneal to 1000 K. Prolonged oxidation transforms the whole film to the thermodynamically more stable Fe3O4(111) phase. This phase transition requires at least two bulk repeat units of Fe3O4(111) and does therefore not occur in very thin FeO(I 11) films. Oxidation of 1-3 ML thick FeO(I 11) films lead to a heterogeneous nucleation of Fe3O4(111) islands, preferentially at step edges where due to iron oxide diffusion the required thickness can locally be established. Homogeneous nucleation of self-assembled, periodic Fe3O4(111) nanodomains embedded in an ultrathin FeO(l 11) occurs in similar to4 ML thick FeO(111) films. The driving force is an electrostatic and thermodynamic energy gain. The results are compared to the Stranski-Krastanov growth of iron oxides on Pt(I 11) [Weiss, W.; Ritter, M. Phys. Rev. B 1999, 59, 520 1]. Differences in the growth behavior of iron oxides on Ru(0001) and Pt(I 11) are discussed in terms of the different affinity of the iron oxide film toward both metals, showing that the stabilization mechanism and growth mode for the same metal oxide phases depends strongly on the substrate.