Triple stage threshold ionization mass spectrometry is successfully implemented in an Ar/C2H2 expanding thermal plasma setup. More than 20 hydrocarbon radicals and molecules formed in the plasma, including for example the C2H, C-3, C3H and C3H3 radicals or C3H4 or C4H2 molecules, are measured and their absolute densities are determined. Thanks to a careful design, a high sensitivity of the instrument is achieved and species with densities as low as 1 X 10(16) m(-3) can be detected. Issues related to the absolute density calibration procedure are considered. The proper determination of the background signal by means of a beam chopper and the influence of the chopper on the measurement are discussed and the possible composition distortion of the sampled beam due to the collisions in the sampling orifice is checked. Furthermore, reported values of electron impact ionization cross sections of hydrocarbon species are compared and, based on their similarity in the near threshold energy region, the C2H2 electron impact ionization cross section is proposed as a reasonable approximation for other hydrocarbon radicals. The results for C, CH, and C-2 radicals are compared with previous cavity ringdown absorption spectroscopy measurements and a good agreement is obtained. (c) 2005 American Vacuum Society.