An automated, tuned Langmuir probe has been applied to a commercial, multiply used reactive ion etcher operated by radio frequency-generated 13.56 MHz low-pressure discharges. The probe was used to measure the plasma parameters and electron energy distribution function in a methane/ hydrogen plasma, an important etchant of optoelectronic semiconductors. Results have revealed a typical electron temperature of approximately 1.5 eV, an electron density of 6X10(9) cm(-3), and a plasma potential of 28 V. The electron energy distribution function is bi-Maxwellian with a dip appearing at 6 eV, which corresponds to the vibrational state of the methane radical and supports a hypothesis of electron absorption by the radical at this energy. The precautions required to condition both probe and chamber to obtain reliable data are given, and it is shown that the automated Langmuir probe can operate as an in situ diagnostic in polymer producing processing plasmas in operational reactors providing stringent routines are implemented.