We used high-resolution scanning tunneling microscopy to study the structure of ultrathin FeO islands grown on Pt(111). Our focus is particularly on the edges of the FeO islands that are important in heterogeneous catalysis, as they host
the active sites on inversed catalysts. To imitate various reaction environments we studied pristine, oxidized, and reduced FeO islands. Oxidation of the FeO islands by O2 exposure led to the formation of two types of O adatom dislocations and to a restructuring of the FeO islands, creating long O-rich edges and few short Fe terminated edges. In contrast, reducing the FeO islands led to a dominance of Fe-rich edges and the occurrence of few and short O-rich edges. In addition, for reducing conditions we observed the formation of O vacancy dislocations on the FeO islands. Through the identification of O adatom and O vacancy dislocations known from closed ultrathin FeO films and geometrical considerations we unraveled the atomic structure of the predominant FeO boundaries of pristine, oxidized, and reduced FeO islands. The results indicate an astonishing flexibility of the FeO islands on Pt(111), since the predominant edge termination and the island shape depend strongly on the preparation conditions.