A systematic study of CO oxidation is performed by considering the LH and ER mechanisms initiated by both molecular and atomic oxygen. In the ER mechanism pure and alloyed clusters present similar barriers and the whole process is downhill. Nonetheless, a pronounced dependence on the Ge concentration is found for the LH pathway, which is the most likely reaction channel. For Pt 4 , the barriers for the CO* + 2 ✕ O* are ca. 1 eV. In addition, pure Pt is prone to become poisoned by CO and CO 2 . On the contrary, the poisoning effects are overcome on PtGe. Pt 4 Ge, Pt 4 Ge 2 and Pt 4 Ge 4 present acceptable reaction barriers except one of the steps in the CO* + 2 ✕ O* route which is blocked with relatively high activation and reaction energies. For Pt 4 Ge 3 , all the reaction barriers are attainable (< 1 eV).