Before moving to the discussion of the CO reaction mechanisms we first study the adsorption of the reactants, viz., CO and O 2 , on Pt 4 Ge n /MgO with n = 0-4. These surface-mounted nanoclusters have been recently characterized and are predicted to be synthetically accessible under Ge-rich conditions. For each composition, various conformers would coexist since they are energetically competitive, except for Pt 4 Ge in which a single isomer, the global minimum (GM), dominates the population. Note that, while all the bimetallic clusters have a closed-shell electronic structure, Pt 4 /MgO possesses unpaired electrons in most cases. Because isomers can exhibit different reactivity, CO and O 2 were adsorbed on the most populated conformers of each Pt 4 Ge n composition. The resulting most stable adsorption geometries along with the CO and O 2 binding energies (BE), Boltzmann populations of each structure at 500K, and Bader effective charges are summarized in Figure . The first thing to note is that after adsorption the size of the ensemble and the relative stability of the isomers changes, showing the dynamic effect of adsorption. CO adsorption has a profound effect on the cluster population. For instance, in Pt 4 Ge 3 , isomer IV in ref (population at 500K of 8.3%) becomes predominant upon CO adsorption, while the most stable bare clusters become negligible. In general, CO prefers to adsorb on Pt through the C end in an atop configuration (∠Pt-C-O 180º), with a few exceptions, where it binds in a bridge fashion. CO adsorption has little impact on the cluster geometry, and CO is never found adsorbed on Ge which indicates the much higher preference towards Pt. O 2 is found to adsorb either dissociatively (2 ✕ O*) or non-dissociatively (O 2 *). Contrary to CO, oxygen can bind to both Pt and Ge, the most common active site being a bimetallic PtGe ensemble. In the dissociative adsorption atomic O binds in a M-O-M bridge configuration, while in the non-dissociative adsorption it usually adopts a M-O-O-M two-member bridge site. It should be noted that in many isomers O* adsorbs at the cluster-support interface bonded to a Mg atom, which is in stark contrast to CO. Moreover, a significant distortion is observed upon adsorption due to the oxophilicity of Ge. In some adducts (e.g., Pt 4 Ge2O isomer I) O* opens the Pt-Ge polar bonds deforming the structure. This effect may become more significant at strong oxidizing conditions.