The waste gases in low-pressure chemical vapor deposition and plasma enhanced chemical vapor deposition semiconductor processes react strongly with residual air in the exhaust of the vacuum pump to form solid deposits. It is common practice to purge the exhaust line with pure nitrogen to reduce the concentration of ambient air between the pump and the scrubber to an acceptable low level. However, the unexpected high quantities of exhaust deposits found in some systems cannot be explained by air leaks, impurities in the nitrogen, or diffusion. The problem of anomalous exhaust deposit formation was examined experimentally. A 40 mm pipe, up to 5 m in length and initially filled with nitrogen, was connected to an 80 m3/h dry vacuum pump exhausting to air at atmospheric pressure. The composition of the atmosphere in the pipe was analyzed with a mass spectrometer. Under static conditions, the gas density distribution was ultimately limited, as expected, by simple interdiffusion. When the pump was running, however, there was a dramatic enhancement of the oxygen concentration throughout the pipe. The experimental observations were interpreted in terms of Richardson’s annular effect. It is shown that the pressure fluctuations typical of vacuum pumps are responsible for the enhanced counterflow of air. The effect can be reduced by applying nitrogen purge but, even with a moderate 20 l/min flow rate, deposits can form due to air transported back from the scrubber or exhaust duct.