Initial studies have been performed to investigate the use of positrons to ionize polyatomic molecules for mass spectrometric analysis. A positron beam facility has been constructed which converts forward-scattered gamma radiation from a 150-MeV accelerator into positrons of a selected energy by means of a tungsten metal moderator. The positrons are transported by solenoidal magnetic fields into the ionization volume of a time-of-flight mass spectrometer where they interact with molecules of a target gas. The initial mass spectral results are apparently due to positron impact ionization and compare directly with electron impact ionization data acquired with the same apparatus. Further studies will seek to extend the energy range of the positrons into the region below the ionization energy of the target molecules, where other mechanisms of ionization are likely.