MgO films were grown on (0 0 1) yttria-stabilized zirconia (YSZ) substrates by molecular beam epitaxy (MBE). The crystalline structures of these films were investigated using X-ray diffraction and transmission electron microscopy. Growth temperature was varied from 350 to 550 degrees C, with crystalline quality being improved at higher temperatures. The MgO films had a domain structure: (1 1 1)[1 1 (2) over bar](MgO)parallel to(0 0 1)[1 0 0](YSZ) with four twin variants related by a 90 degrees in-plane rotation about the [1 1 1](MgO) axis. The observed epitaxial orientation was compared to previous reports of films grown by pulsed laser deposition and sputtering and explained as resulting in the lowest interface energy. (C) 2008 Elsevier B.V. All rights reserved.