We have investigated on the response of two porous silicon samples, and doped, to oxygen. Both the samples exhibit a quenching following the Stern-Volmer model. Time resolved PL measurements have been also carried out in order to measure the carrier lifetime in absence of the quencher. The reactivity rate constant has been computed and it has been found to depend, for sample, on the characteristic nano-dimension of the emitting structure. This suggest that chemical reactivity is strictly correlated to the confinement size.