Ultrathin iron oxide films are useful model materials for fundamental studies of surface processes and exhibit intriguing properties as catalysts, as demonstrated recently in a number of studies utilizing platinum as a substrate. We report a study of the initial stages of iron oxide film growth on an Ag(100) surface using scanning tunneling microscopy, low energy electron diffraction, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy, with the goal of elucidating the effects of the substrate material on FeO x film growth and physical properties. We demonstrate that a well-ordered, monolayer-thick FeO(111) film can be prepared which is similar to the well-studied structure formed on Pt (111), though with a significantly expanded lattice constant indicative of smaller Fe−O buckling and weaker interactions with the substrate. Increased oxygen pressure during deposition leads to formation of a multilayer phase taking the form of well-ordered islands. Although superficially similar to FeO(111), spectroscopic measurements show a substantial proportion of Fe 3+ in the phase. FeO(100) grains are observed upon deposition at elevated substrate temperatures, which is proposed to result from formation and oxidation of iron clusters embedded in the surface.