Half-metallic ferrimagnetic materials such as Fe3O4 are of interest for use in spintronic devices. These devices exploit both the spin and charge of an electron in spin-dependent charge transport. Epitaxial thin films of Fe3O4 have been grown on the three low-index planes of gold by electrodeposition. On Au(110), a [110] Fe3O4 orientation that is aligned with the underlying Au(1 10) substrate is observed. Thin films on Au(100) grow with three different orientations: [100], [111], and [511]. On Au(111), both [111] and [511] orientations of Fe3O4 are observed. The [511] orientations are the result of twinning on {111} planes. A polarization value of approximately -40% at the Fermi level was measured by spin-polarized photoemission at room temperature for a thin film on Au(111).