The reaction of Ar+ with NO to yield the possible ground-state NO+(X1Σ+) and metastable-state NO +(a 3Σ+) product ions has been studied near 300 K in a flow-drift tube using helium and argon buffer gases separately. The "apparent" rate constant obtained in the argon buffer is an order of magnitude smaller than the rate constant obtained in the helium buffer. The difference in the rate constants is due to the "back" reaction of the NO+(a 3Σ+) product ions with the argon buffer-gas atoms to yield the possible product ions NO+(X 1Σ+) and Ar+. The ratio of the rate constants places precise limits on the product ratios in both cases; namely, the Ar++NO reaction yields greater than 85% NO+(a 3Σ+) ions and the NO+(a 3Σ+)+Ar reaction yields greater than 85% Ar + ions. The data also show that the rate constant for the quenching of NO+(a 3Σ+) ions by NO is (8.0±4.0)×10-10 cm3/s at 300 K.