We have studied the effect of the composition on the CO oxidation reaction catalyzed by Pt 4 Ge n clusters (n = 0-4) supported on MgO(100) using a combination of DFT and microkinetic simulations. To characterize the most stable CO and O 2 adsorption configurations, a global minima search was carried out on the thermally populated isomers of each cluster composition. The DFT results reveal that CO binding energy weakens with the Ge content owing to ligand (electronic) effects induced on Pt upon Ge alloying. Bond-length and PDOS analysis indicate that O 2 is readily activated up to the peroxo state and dissociates to atomic oxygen with low barriers and significant release of energy in all the Pt 4 Ge n clusters.