Molecular beam deposition systems allow for unparalleled control of film composition and structure. This article addresses the capacity for controlling metal and oxidant fluxes in the Yb/O-2 system to access the metastable phase ytterbiurn monoxide (YbO). Experiments exploring the growth of polycrystalline YbOx films by molecular beam deposition demonstrate that a 2:1 molar ratio of Yb:O-2 fluxes is necessary to achieve preferential growth of the divalent oxide. Applying similar deposition conditions to a (0 0 1) GaN surface leads to the growth of epitaxial (1 1 1) YbO films. Similar to other rocksalt oxides grown on GaN surfaces, YbO films display a 3D growth mechanism that leads to a grainy morphology with crystallites of 50 nm lateral dimensions. Rocking curves in omega and phi have full-width half-maximum values of 1.77 degrees and 4.1 degrees, respectively; further improvements in crystal quality appear to be limited by the thermal stability of the YbO phase. (C) 2007 Elsevier B.V. All rights reserved.