Silver containing zeolites are widely used in heterogeneous catalysis and their catalytic performance is strongly dependent on the nature of the metal species supported on the zeolites. In this work, Ag-ITQ2 zeolites have been prepared by different methods: incipient wetness impregnation, ion exchange and preformed Ag-nanoclusters supported on the zeolite. The samples have been pretreated with oxygen or hydrogen previously to the catalytic test in the CO oxidation reaction. The results allow to correlate the nature of the Ag-species with its catalytic activity in this reaction as metallic silver is active for the CO oxidation but cationic silver does not show significant activity. It is shown that Ag-0 is formed on the catalysts prepared by impregnation even without a reduction treatment. On the contrary, Ag-nanoclusters form stable partially charged silver species that only can be reduced to Ag-0 nanoparticles under severe thermal conditions. Silver species formed in the catalysts prepared by ion exchange also present a partial positive charge. We concluded that this reaction is a useful test for monitoring the nature and evolution of silver species in the zeolite and to determine the stability of the silver nanoclusters.