Using vibrationally resolved ultraviolet photoelectron spectroscopy, we demonstrate that O-2 molecularly adsorbs on Cu-6(-) at room temperature. On Cu-6(-), we found that dissociation Of O-2 is energetically feasible, but kinetically hindered. These results are unexpected on the basis of the previous observations that O-2 dissociatively chemisorbs on Cu bulk surfaces as well as Cu monomer and dimer anions, which can lead to the formation of Cu oxide. By changing the sizes of nanoclusters atom by atom, one can observe new chemical properties, which are not expected on the basis of the results from atom, diatomic molecule, and respective bulk materials consisting of the same element.