The bimetallic nanocluster catalyst structure can change during pretreatment and reaction, thus in situ characterization techniques are required for a proper analysis of the active sites. In situ XAFS and DRIFTS were used to study the dynamic evolution of the metal active sites in bimetallic AgxAu25-x nanoclusters supported on ITQ2 zeolite during CO catalytic oxidation. The activity of the bimetallic nanocluster catalyst in this reaction was significantly higher than those of supported monometallic Ag25 and Au25 nanoclusters. These results were explained by the formation of AgAu alloyed nanoparticles, which favoured reactant adsorption and reaction. Furthermore, the initial activity depended on the catalyst pretreatment, obtaining better conversion, at lower temperatures, with the catalyst pretreated with hydrogen than with the catalyst pretreated with oxygen. This was also associated with an easier formation of a AgAu alloy under hydrogen pretreatment at 150 °C. However, the alloying process seemed to be completed after reaction in both cases, i.e. for the catalyst pretreated with oxygen and with hydrogen, obtaining the same catalytic performance with both catalysts upon reuse. The activity is constant in successive reaction runs, indicating high stability of the active species formed under reaction conditions. The results have shown that the combination of catalytic studies with in situ characterization techniques provides insight into the structural dynamics of the catalysts during activation and reaction.