Thin Fe3O4 films prepared by iron deposition and subsequent oxidation on Pt(111) single crystal substrates were studied by selected area electron diffraction and high-resolution transmission electron microscopy (HRTEM). No other iron oxide phases were detected. The formation of Fe3O4 films takes place epitaxially on Pt(111) substrates with the relationships: [111](Pt)/ /[111]Fe3O4, [1 (1) over bar0]Pt/ /[1 (1) over bar0]Fe3O4. The films were free of dislocations but contained antiphase boundaries (APB) between domains shifted by 1.86 or 2.35 A relative to each other along the [111] direction. The lattice mismatch between Fe3O4 and Pt causes periodic arrays of strained regions in the oxide along the interface. The iron oxide lattice parameters near the interface are compressed by approximately 2%. Detailed analysis of the Fe3O4/Pt interface based on HRTEM images and image simulations show that the first layer of the oxide on the Pt substrate consists of iron atoms.