Structures of coinage metal clusters reacted with atomic and molecular oxygen were studied using Ultraviolet Photoelectron Spectroscopy and Density Functional Theory calculations. We show that O-2 partially dissociates on Ag-2(-), and this dissociative chermsorption is a kinetically hindered step. For Au4O2-, in addition to the previously observed molecularly adsorbed oxygen, we are now able to synthesize a second isomer using atomic oxygen reagents, in which oxygen adsorbs dissociatively. We demonstrate that different isomers can be distinguished by comparative studies of chemisorption of atomic and molecular species on metal clusters, which is otherwise difficult. (C) 2004 Elsevier B.V. All rights reserved.