In order to characterize the oxidation of metallic surfaces, the reactions of O-2 with a number of Al-x(-) and, for the first time, Ga-x(-) clusters as molecular models have been investigated, and the results are presented here for x = 9-14. The rate coefficients were determined with FT-ICR mass spectrometry under single-collision conditions at O-2 pressures of similar to 10(-8) mbar. In this way, the qualitatively known differences in the reactivities of the even- and odd-numbered clusters toward O-2 could be quantified experimentally. To obtain information about the elementary steps, we additionally performed density functional theory calculations. The results show that for both even- and odd-numbered clusters the formation of the most stable dioxide species, [MxO2](-), proceeds via the less stable peroxo species, [M-x(+)...O-2(2-)](-), which contains M-O-O-M moieties. We conclude that the formation of these peroxo intermediates may be a reason for the decreased reactivity of the metal clusters toward O-2. This could be one of the main reasons why O-2 reactions with metal surfaces proceed more slowly than Cl-2 reactions with such surfaces, even though O-2 reactions with both Al metal and Al clusters are more exothermic than are reactions of Cl-2 with them. Furthermore, our results indicate that the spin-forbidden reactions of O-3(2) with closed-shell clusters and the spin-allowed reactions with open-shell clusters to give singlet [M-x(-)...O-2(2-)](-) are the root cause for the observed even/odd differences in reactivity.