For some plasma experiments high purity hydrogen or deuterium at pressures of 10−3Torr or less is required. It is shown that a molecular pump can be used as a combination pumping, gas injection, and gas purification system. The gas is admitted to the fore vacuum side and streams back to the high vacuum side. This does not materially affect the pumping of heavy impurities. Performance data for this mode of operation are given for the Pfeiffer pump TVP‐500.