Experimental evidences for the non-dissociative chemisorption of O-2 are presented on even-numbered free Au anion clusters (Au-n(-), n =number of atoms) up to Au-20(-) at room temperature. Our result indicates that the formation of the activated di-oxygen species is the key of the unusual catalytic activities of Au-based catalysts. No correlation between geometrical structures of Au-n(-) and the activities towards O-2 adsorption was found, showing that site-specific chemistry disappears for Au-nanocatalysis. We demonstrate that interplay between cluster physics and surface chemistry is a promising strategy to unveil mechanisms of elementary steps in nanocatalysis. (C) 2003 Elsevier B.V. All rights reserved.