The collection efficiency of a plane acting as a pump in a vacuum system can be determined by measuring the rate at which molecules of a given species strike and the rate at which they return from that plane. This paper discusses an experimental vacuum system incorporating a movable partial-pressure analyzer capable of making such measurements. The experimental procedure discussed here provides a means for evaluating parameters affecting the pumping efficiency. Efficiency values obtained by the present approach are compared with values obtained from kinetic theory and from speed-factor measurements. The experimental data show that much of the deviation of reported pumping efficiencies can be explained by fragmentation of the test-gas molecules and by a pressure anisotropy in the test dome. A means of calibrating the analyzer with vapor-pressure data is also discussed.