Gas phase kinetics experiments with mass-selected gold, silver, and binary silver-gold cluster ions in a radio frequency (rf)-ion trap reactor are presented. For the catalytic CO combustion reaction, the adsorption of molecular oxygen onto the free clusters is identified as the first reaction step. A comparison of the measured O-2 adsorption reaction rate constants for the investigated AgnAum- (n + m = 1, 2, 3) clusters reveals, a pronounced size and composition dependence. Favorable activation of the oxygen molecular bond is only expected in the case of Au-2(-) . In the reaction of the gold diner with O-2 and CO, a coadsorption complex is identified at cryogenic temperatures as decisive reaction intermediate in the observed CO oxidation reaction. Through detailed reaction kinetics measurements in combination with first-principles calculations, a comprehensive understanding of the molecular details of the catalytic reaction cycle emerges. The obtained data also permit the determination of the CO2 formation rate and the catalytic turn-over-frequency in the rf-ion trap reactor. In contrast to gold, odd size silver cluster anions are found to adsorb two oxygen molecules. These AgnO4- complexes are proposed to be key intermediates oxidation reactions with silver clusters involved, and first experimental indications for catalytic activity of selected cluster sizes are presented. (c) 2005 Elsevier B.V. All rights reserved.