The reactivity of Mg epilayers on Si(111)-7 × 7 towards molecular oxygen is investigated as a function of the metal film thickness in the range between 7 and 45 monolayers. Quantum well and surface states are characterized with ultraviolet photoelectron spectroscopy demonstrating the epitaxial and single-crystalline structure of the Mg films. The oxidation rate is monitored during the reaction by measuring chemicurrents at 110K in the Mg/p-Si(111) Schottky diodes due to the non-adiabatic character of at least one step in the reaction chain. For film thicknesses around 9 and 13 monolayers the chemicurrent transients demonstrate that the reaction rate is strongly enhanced by a factor of more than two. With Mg 2p core level spectroscopy, a similar enhancement can be found for the total oxygen uptake for long exposures indicating that the chemicurrent increase measures solely a quantum size effect on the reactivity and no device-related effects. The enhanced reactivity can be explained by the increased first charge transfer into the affinity level of the approaching molecule when a quantum well state appears at the Fermi level and increases the density of electronic states. A linear relationship between the photoelectron intensity at the Fermi level and the maximum chemicurrent is clearly observed. On the other hand, the surface work function and the Schottky barrier height exhibit almost no correlation with the enhanced reactivity.