The chemical compositions of 4% carbon in hydrogen/deuterium electron cyclotron resonance (ECR) microwave plasmas based on ethane, ethylene, acetylene, and methane, as determined by supersonic pulse, plasma sampling mass spectrometry, have been kinetically modeled using only a steady-state concentration of hydrogen to represent the role of the plasma. Using 375 isotopically labeled chemical steps based on 54 reversible neutral molecule chemical reactions, simulated spectra were generated that matched all eight experimental spectra using only literature values of the kinetic and energetic constants and three physically reasonable fitted parameters that were held constant for all eight simulations. The success of the modeling provides strong evidence that the chemistry of ECR-microwave plasmas is dominated by neutral molecule reactions. (c) 2006 American Vacuum Society.