Reactions of CO and O2 on size-selected copper cluster anions, Cu-n (n = 4-11), have been investigated at the collision energy of 0.2 eV by use of a guided ion beam-tandem mass spectrometer. Oxygen-adsorbed copper anions, CunO-2, in particular Cu5O-2 and Cu9O -2, show an evidence of the CO oxidation, that is, the formation of the monoxide CunO-. The density functional theory calculation reveals that the CO oxidation occurs more exothermically on Cu5O-2 and Cu9O- 2 than the other clusters. This can be explained by the relatively small dissociation energy of their Cu-O bonds. In addition, the calculations on Cu5O+2/- indicate that the CO oxidation proceeds via a low-energy pathway for the anion owing to the structural rearrangement of the copper cluster compared to the cation.