Reactions of free silver anions Ag-n(-) (n=1-13) with O-2, CO, and their mixtures are investigated in a temperature controlled radio frequency ion trap setup. Cluster anions Ag-n(-) (n=1-11) readily react with molecular oxygen to yield AgnOm- (m=2, 4, or 6) oxide products. In contrast, no reaction of the silver cluster anions with carbon monoxide is detected. However, if silver cluster anions are exposed to the mixture of O-2 and CO, new reaction products and a pronounced, discontinuous size dependence in the reaction behavior is observed. In particular, coadsorption complexes Ag-n(CO)O-2(-) are detected for cluster sizes with n=4 and 6 and, the most striking observation, in the case of the larger odd atom number clusters Ag-7(-), Ag-9(-), and Ag-11(-), the oxide product concentration decreases while a reappearance of the bare metal cluster signal is observed. This leads to the conclusion that carbon monoxide reacts with the activated oxygen on these silver clusters and indicates the prevalence of a catalytic reaction cycle. (C) 2004 American Institute of Physics.