Here, we investigate p-type and n-type porous silicon (PS) reactivity towards oxygen and ammonia in the gas phase. We show that in both the environments, a clear correlation exists between the reaction rate and the confinement energy. In the frame of Marcus theory and starting from basic assumptions, a theoretical model has been developed correlating the charge transfer mechanism to the confinement size. The model well explains the experimental findings, predicting, moreover, the existence of a resonance in the reaction rate, function of the confinement energy.