We used a quadrupole mass spectrometer (QMS) to investigate the dissociation of source gases and the generation of radical species during the hot-filament chemical vapor deposition (HFCVD) process at various methane and ammonia concentrations. Since the HFCVD was carried out at the working pressure of 30 Torr, a capillary tube and a 5 mum pinhole were installed to sample the small amount of gas from the HFCVD chamber to the differentially pumped QMS at the vacuum level of 5 x 10(-6) Torr. An investigation of the systematic variations in mass spectra corresponding to the increase of methane concentration in the ammonia-free source gases revealed substantial generation of chemical species containing two carbon atoms. However, an increase of ammonia concentration in the source gas with 20% of methane resulted in generation of nitrogen containing radicals such as CN and HCN, together with the suppression of two-carbon radicals. An implication of observed mass-spectrum variation for the growth behavior of carbon nanotube and/or carbon nanoparticle films was discussed. (C) 2004 American Vacuum Society.