Thin oxide films epitaxially grown on metal single crystal surfaces may exhibit structural properties that differ from the corresponding bulk oxide materials. The structure of the films is often rendered by their thickness, the structure and properties of the substrate and by the nature of the oxide/substrate interface. We prepared thin iron oxide films on Pt(111) and Ru(0001) and studied their structure using STM, LEED and XPS. The structure of FeO(111) – the iron oxide phase that forms at the interface with the metal single crystal – depends on the parameters of the support and is believed to further influence the structure of thicker iron oxide films, such as Fe3O4(111), that are being grown on top of it. In this article we discuss the role of one of the important parameters that may determine this structure – the epitaxial strain at the iron oxide/metal single crystal interface.