The (010) surface of single crystal MoO 3 has been prepared and examined using LEED, XPS, UPS, and ELS. Three methods yield the stoichiometric surface: scraping in UHV and annealing, ion etching followed by reoxidation (770 K, 10 2 Pa O 2), or oxygen treatment to remove carbon contamination. LEED shows the surface periodicity is the same as that of the bulk (010). The MoO 3 valence band is 7 eV wide with density of states maxima at 1.5, 3.6, and 5.6 eV below the top of the valence band. Heating MoO 3 in vacuum reduces the surface region. XPS indicates the O/Mo atomic ratio decreases to 2.85 ± 0.12 on heating to 600 K. Ar ion bombardment disorders the surface and reduces the surface O/Mo atomic ratio to 1.6. Annealing of reduced surfaces at > 770 K incompletely reoxidizes them by diffusion of oxygen from the bulk. UPS of reduced and annealed MoO 3 exhibits two new emission features in the bandgap at 0.9 and 2.0 eV above the top of the valence band. These features originate from Mo derived states of a defect involving two or more Mo atoms, such as crystallographic shear planes. Because of the insulating nature of MoO 3, surface charging and electron beam induced damage were substantial hindrances to electron spectroscopic examination.