Ultrathin MgO films have been synthesized under ultrahigh vacuum conditions by evaporating Mg onto Mo(100) in various background pressures of oxygen. Low-energy electron diffraction and surface spectroscopic studies show that the MgO films, prepared under optimum oxidation conditions, grow epitaxially in the 200–600 K substrate temperature range and have an essentially one-to-one stoichiometry. The nature of the near-surface defects of the MgO films grown at low oxygen pressures has been explored using electron energy-loss spectroscopy. Finally, the chemical properties of the stoichiometric MgO films have been investigated using high-resolution electron energy-loss spectroscopy.