We present a method for detecting and quantifying the concentration of free radicals produced in a low temperature methane process plasma employing quadrupole mass spectrometry and Bayesian analysis. The latter includes a model comparison to determine species expected in the plasma and a parameter estimation of their concentrations and cracking coefficients. We find that the neutral mass spectra are best explained by incorporating the neutral molecules H-2, CH4, C2H2, C2H4, and C2H6 as well as the radicals H, CH3, and C2H5 into the model. (C) 2003 American Vacuum Society.