After O 2 chemisorption, the cluster activates the O-O bond by transferring electrons to the antibonding 2π* orbitals. For the bridge adsorption mode O 2 is activated in the peroxo state where the IS structure is close to the transition state (TS). The cluster continues to stretch the bond until finally the O-O scission is achieved. In all cases the O-O bond is already cleavaged in the TS, even for the atop adsorption (see Figure ). Our calculations reveal that the O-O bond expands from ~1.47 Å to a value in the range of 1.90-2.15 Å in the TS. Here the charges of the O atoms are maximal; each atom carries 0.6-0.8 |e| comparing to ~0.4 |e| in the IS. The values of these charges are similar for adsorption on Pt-Pt and Pt-Ge of M-O-O-M bridge sites. Afterwards, the O atoms separate and occupy a bridge site in the vicinity. This process is found to be exothermic, the greatest energy release occurring when the strong Ge-O bonds distort the cluster structure (e.g., Pt 4 Ge 4 isomer II, Figure ). Overall, the activation barriers are small and most cases lie in the range of 0.4-0.6 eV, although there are some exceptions where the barrier increases to 0.75 eV, as for the Pt 4 Ge 3 isomer I (see Figure ). Our results indicate that at room temperature most of the adsorbed oxygen will be in the form of atomic O*.