The ionization potential (IP) of CunO2 clusters (n = 15-46) has been m
easured and the shift in IP induced by the adsorption of the O-2 molec
ule evaluated. There are no large discontinuities in the IP of CunO2,
in contrast with the electronic shell closings observed for pure Cu-n,
but the even-odd alternation in the IP persists upon oxidation. On av
erage, the IP increases after oxidation, with larger shifts for cluste
rs where the pure Cu-n have low IP (n = 21, 41) and small or negative
shifts for clusters originally having high IP (n = 30, 34, 40).