The sputtering threshold energy and low‐energy sputtering yields for films of sputtered quartz and Pyrex were measured with Langmuir probes. The probe characteristic is very sensitive to deposits of insulating material because even very thin insulating films cause radical changes in electron acceptance. A thin insulator film of known thickness is deposited on the probe by rf sputtering, and a known fraction of that film is removed by enforcing a dc bias voltage on the probe. Different bias voltages require different times to remove the same amount of material (as monitored in the probe characteristic), and one can use this as a very sensitive method for collecting sputtering yield data. Analysis of the deposit sputtered from a quartz target in a 10−2Torr Ar discharge shows it to be SiO1.6. The sputtering yield curve for this SiO1.6is presented and shows a yield of ∼0.1 molecules/ion at 100 eV with a sputtering threshold of ∼16 eV.