The combination of low-energy electron diffraction intensity analyses and scanning tunneling microscopy was used to investigate the morphology and atomic structure of thin Fe films grown on Au(100) at 400 K. Deposition of only about 0.2 monolayers (ML) Fe is sufficient to lift the reconstruction of the clean substrate. In the initial growth process (<~1 ML) place exchanges between Fe and Au lead to almost two-dimensional subsurface Fe film growth with one layer of Au covering the entire film. This way, gold acts as a “self-surfactant.” Yet, there are deviations from two-dimensional growth, with a second Fe layer beginning to grow before the first one is fully completed and some substitutional disorder developing in the film because of incomplete place exchange. The amount of gold floating on the surface only gradually decreases with further increasing film thickness. At about 2 ML the surface undergoes a complete restructuring during which short “wormlike” chains of atoms form and long-range order is destroyed. Nevertheless, the existence of large terraces of little roughness proves that some surface activity of gold remains. At coverages of several ML, long-range order is reestablished with the Fe film growing in an undistorted bcc arrangement. Although parts of the film are still covered by gold, the surface morphology is not very different from that known for homoepitaxial growth of Fe on Fe(100), i.e., gold has stopped to serve as a “self-surfactant.”