The synthesis of ytterbium monoxide (YbO) by molecular beam deposition (MBD) is investigated for developing a lattice-matched oxide to gallium nitride. Because Yb2O3 is the thermodynamically stable oxide at atmospheric pressure, YbO has previously only been prepared by high-pressure bulk synthesis reactions. This article demonstrates that through low growth temperature and the kinetic control of molecular fluxes allowed by MBD, the polycrystalline monoxide phase can be stabilized in thin film form on silicon substrates. Once deposited, the YbO rocksalt structure is found to be stable at room temperature and atmospheric pressure. Elemental analysis indicates that films are primarily composed of Yb and O. Elemental concentrations of H, C, and N, which are known to form phases with Yb of similar structure and lattice parameter to YbO, are shown to be below the 1 at. % level. (c) 2006 American Vacuum Society.