Experiments using vibrationally resolved ultraviolet photoelectron spectroscopy show that oxygen selectively adsorbs on even-numbered Ag anion clusters, whereas the odd-numbered clusters are not chemically active. We provide evidence that O-2 molecularly adsorbs on Ag anion clusters consisting of less than 15 atoms. O-2 adsorption pattern for Ag anion clusters is quite analogous to that of Au anion clusters. We suggest that for the reactions efficiently catalyzed by Au-nanocatalysts, Ag nanoclusters can be a promising candidate as a building block of catalysts. (C) 2003 Elsevier B.V. All rights reserved.