We have investigated the structural and compositional changes that are induced by the segregation of substrate Mg to the surface of 1μm-thick Fe3O4 films on MgO(001). The thin films have been grown with plasma-assisted MBE, and characterization with RHEED (reflection high-energy electron diffraction), x-ray diffraction (XRD), and Superconducting Quantum Interference Device (SQUID) magnetometry show slightly strained, single-crystalline Fe3O4 films. For the surface studies, we have combined Low-Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM). Initial and final surface characterization employed X-ray Photoelectron Spectroscopy (XPS) and Ion Scattering Spec-troscopy (ISS) respectively. The surfaces of the MBE-grown samples are flat and show a (√2 × √2)R45° reconstruction with respect to the Fe3O4 surface unit cell. We observe the onset of Mg segregation to the surface at around 700 K, with long, narrow extensions of terraces being observed growing along the  and  directions. Upon prolonged heating at 800 K, massive Mg segregation to the surface is observed. Heating in an oxygen atmosphere induces a 1×4 surface reconstruction, and results in extremely long (≈ 1000 Å), wide terraces.