A catalytic redox reaction involving N₂O and CO on size-selected copper cluster anions, Cu_n⁻, was investigated in the gas phase using a guided ion-beam tandem mass spectrometer. When Cu_n⁻ is exposed to a mixture of N₂O and CO, Cu_nO⁻ is produced via the decomposition of N₂O. Increase of the CO partial pressure results in the reproduction of Cu_n⁻ and decrease of Cu_nO⁻ through the oxidation of CO. The present results demonstrate that a full catalytic cycle for the reaction, N₂O + CO → N₂ + CO₂, takes place on copper cluster anions. Furthermore, in the investigations of the elementary reactions of Cu_n⁻ + N₂O and Cu_nO⁻ + CO, we found that the catalytic oxidation of CO with N₂O on Cu_n⁻ proceeds most efficiently at n = 7 in the size range of n = 5–16.