Performance, plume properties, and discharge stability of a 9 cm diameter laboratory model Hall thruster were measured at vacuum chamber background pressures ranging from 1e-6 to 6e-5 Torr. The thruster was operated at constant discharge powers of 300 and 500 W while background pressure was varied from the minimum achievable value to values 10, 20, and 40 times greater than the minimum at each power level. Discharge current oscillation amplitude generally increased with increasing background pressure, and the sensitivity of oscillation amplitude to background pressure increased with increasing applied magnetic field strength. The fundamental frequency of the discharge current oscillations varied with background pressure. Thrust, specific impulse, and efficiency declined as stability degraded, primarily as a result of increased electron cross-field mobility, broadening of the ion energy distribution, and increased multicharged ion production. Ingestion of background neutrals appeared to lengthen the ionization zone and shift it toward the anode.