Low sputtering rates are measured by means of the probe technique in a plasma. The displacement of the probe curve along the voltage axis (and its change in shape) when the probe is covered with a material of different work function, sputtered from a sputtering electrode, is a dependable measure for the thickness of the sputtered layer. The method is very sensitive, generally applicable, and was applied especially to answer the question of a threshold voltageU0for sputtering. The sputtering rate for Pt in Xe, in the region of low ion energies (50<Us<200 ev) is roughly proportional to (Us−U0)2withU0=40 ev.