Methyl radicals from a methane electron cyclotron resonance plasma are measured quantitatively at the sample position by ionization-threshold mass spectrometry (ITMS). The absolute fluxes are determined by calibrating theCH3ITMS results with those of methane, taking into account the published energy-dependent cross sections for the ionization ofCH3andCH4,respectively. The measuredCH3radical fluxes are on the order of some1015 cm−2 s−1, which is in accordance with recent modeling results. ©1998 American Institute of Physics.