The results of the determination of the pumping speed employing conventional methods deviate from the productACefbased on concepts of the kinetic theory of gases, whereACe=Aυav/4is the conductance of an orifice, if the capture probabilityfis not small. This deviation may be attributed to the technique of pressure measurement. In order to obtain proportionality between capture probability and speed with a unique constant of proportionality in the whole range0≤f≤1it is recommended that the pumping speed be defined using the pressure in a large vessel at a place where the isotropic state is maintained to a high degree. This speed is named intrinsic speed and it may be measured using Knudsen's large chamber fitted with a proper gas inlet system. The measurement of very large pumps according to this definition however may be carried out by applying a conversion for the incidence density in the pump mouth to an isotropic pressure. The considerations presented in this paper are intended as a contribution to an international standardization of the speed measurement of high-vacuum pumps. They are based on molecular flow in the test dome.