Various mixtures co nta inin g co mbinations of CO2, O2, or CO have been photoionized at 16.7 and 21.2 eV at pressures up to 1.5 torr in the NBS high pressure photoionization mass spectrometer. In CO2-C O mixtures the interactions of CO; ion s eventually lead to the formation of (COh+ and [(CO),. C02J+ clu ster ions, while photoionization of CO2 -CO-02 mixtures yields mainly oxygen-co ntainin g clusters at higher pressures. The investigation of 0 2'CO mixtures also revealed reactions between Ot and CO. The role of impurity reactions involvin g H2 0 is consid ered in some detail , and th e im plications of all of these data to th e vapor phase radiolysis of CO2 is discussed.