The pumping speed of a sputter-ion pump for hydrogen may vary over a wide range. At procures below 10−8 Torr the speed decreases rapidly and cannot be explained as a simple decrease in the discharge current. In the pressure range between 10−8 and 10−6 Torr the pumping speed for pure hydrogen is of the same order as that for nitrogen. However, if a second gas having a higher sputtering yield for titanium is also present in sufficient concentration, increases of at least a factor of two are observed in the pumping speed for hydrogen. Finally, at hydrogen pressures above 10−6 Torr the long-term pumping of pure hydrogen can produce a marked increase in speed. The extent of this increase is reduced by the presence of active gases such as nitrogen, indicating that change of the rate of diffusion of hydrogen into the titanium at room temperature is the principal factor involved.