Epitaxial Mg-Fe-O spinel thin films were prepared by solid state reactions between MgO(lOO) substrates and thin films or vapors of iron oxide. Iron or iron oxide was deposited by electron beam evaporation in an oxygen background pressure onto a heated MgO crystal. The formation of MgFe2O4 at high temperatures proceeds via cation counter-diffusion in the fixed oxygen sublattice. Depending on substrate temperature, oxygen partial pressure and target material, epitaxial spinel films of different composition and magnetic properties were obtained. Crystal structure, composition and sub-micron morphology were investigated by RBS, XRD, TEM/SAED and EDX. Magnetic hysteresis loops were recorded at room temperature using the magneto-optic Kerr effect. Thickness interference fringes observed by XRD confirm the growth of smooth films. The main feature found in TEM plane view samples is a network of cation antiphase boundaries. The film composition measured by RBS varied from Fe2+ɛO3 films deposited at 340 °C substrate temperature to a solid solution of Fe2+E03 and MgFe2O4 at 500 °C to MgFe2O4 above 800 °C. EDX line-scans show a single phase MgFe2O4 spinel for a substrate temperature of 850 °C, without variation of composition with position or depth. At substrate temperatures higher than 800 °C Fe2+ was additionally dissolved in the MgO substrate, forming a FexMg1-xO solid solution.