Studies have been made in a mass spectrometer at ionization chamber pressures of about 160 μ on mixtures of rare gases, R=He, Ne, Ar, Kr, and Xe, with N2 or CO. The ions R2+, ArN2+, KrN2+, XeN2+, ArCO+, KrCO+, XeCO+, N4+, and C2O2+ were observed in these mixtures as a result of reactions of excited rare-gas atoms. The addition of Ar to N2 enhances the formation of N4+ by reactions initiated by excited states of Ar; the addition of Ar or Kr to CO promotes the formation of C2O2+ through reactions initiated by excited states of Ar and Kr. Rate constants were determined for the reactionsArN2++N2=N4++Ark=1×−10cc/molecule−secKrCO++CO=C2O2++Krk=3×10−10cc/molecule−secat a repeller field strength of 50 V/cm. Other combinations of rare gases with N2 or CO show no such promotional effect on the formation of N4+ or C2O2+. Ratios of the rate constants for the formation of RN2+ and R2+ in rare-gas-nitrogen mixtures and for the formation of RCO+ and R2+ in rare-gas-carbon monoxide systems were also determined. At high pressures of N2 or CO alone the formation of N4+ or C2O2+ is observed, and the reaction is a bimolecular chemi-ionization involving highly excited N2 or CO. At still higher pressures these ions are formed by termolecular ion—molecule processes. At sufficiently high electron energies there are formed in binary mixtures of N2 or CO with different rare gases the following ions: RC+, RN+, C2O+, CO2+ and N3+. The N3+ and RN+ ions are formed by the reaction of excited nitrogen molecule ions with R and/or N2, and the relative rates of these reactions were determined. No compound ions between He or Ne and N2 or CO were observed under any of the conditions of these experiments.