It has been observed that anomalously high residual background-gas pressures will occur in an ultrahigh-vacuum mass spectrometer exposed to increasing argon pressure. The gases have been identified as hydrogen, helium, neon, carbon monoxide, methane, and ethane. The vacuum system is bakeable, with sputter-ion and cryogenic sorption pumps. Vacua of less than 1×10−10 Torr are routinely obtained. A mechanism is described to account for the appearance of the above gases based on the vacuum-system components. The gases either evolve from the sputter-ion pump or are inefficiently pumped by it. These gases migrate through the system and are adsorbed on the various internal surfaces when the pressure is below 10−9 Torr. They are subsequently desorbed by argon bombardment, most likely in the vicinity of the total- and partial-pressure gauges, as the argon pressure is increased. In a static system, the residual gases will increase to as ranch as 1% of the argon pressure. In a dynamic system, the gases are still observed, but at lower concentrations.