The composition of a sealed discharge in He-N2-CO2and He-CO2mixtures was studied with a quadrupole mass spectrometer. A modulated molecular beam sampling technique allowed direct measurement of the molecular, atomic, and ionic species produced in the discharge. Effective formation and destruction rates were measured for various species. In particular, the nitrogen oxides, NO and NO2, are formed in sufficient quantities to affect the performance of CO2lasers. In the He-N2-CO2mixture the dominant ion changes from CO-2to NO+, while in He-CO2, O+2becomes the major ion.