Hydrogen pumping speeds in sputter-ion pumps are particularly dependent upon operating conditions. At high pressures of hydrogen, maximum speeds are obtained with the gas diffusing rapidly into the cathodes, but since the diffusion is sensitive to surface contamination, relatively low partial pressures of gases such as nitrogen can greatly reduce the speed. High hydrogen loads can cause serious cathode deformation by titanium hydride formation and, if high cathode temperatures develop, the reversible desorption of hydrogen can stall the pump. At lower pressures cathode surface contamination becomes of greater importance and the hydrogen pumping speed declines drastically. Because hydrogen is a dominant gas at the ultimate of a metal system, it is necessary to combine either a sublimation or a getter pump with the sputter-ion pump in order to maintain efficient pumping. The influence of hydrogen diffusion rates on the pumping capabilities of titanium and other sputter-cathode and getter materials is reviewed.