The data on adsorption isotherms of4Hein synthetic adsorbents are obtained at 4.2 K in the pressure range1–10−2 torr.The dependence of pressure on the amount of adsorbed gas in adsorption pumps is studied under conditions simulating the pump operation in dilution refrigerators. It is found that the pressure in the pump is practically independent of the amount of adsorbed helium for a constant pumping rate in the interval10−6−10−4 mole/s,over a wide pressure range. The obtained characteristics can be used for designing and developing pumps for dilution refrigerators. ©1998 American Institute of Physics.