As-anodized porous silicon (PS) was investigated in situ by photoluminescence (PL) during the adsorption of H2O, O-2, C2H5OH and C-2(CN)(4) molecules. The PL intensity was increased during H2O adsorption (to 130%) and decreased by adsorption of O-2 (to 30%), C2H5OH (to 22%) and C-2(CN)(4) (to 5%). A strong blue shift was observed during the C2H5OH adsorption. The reversibility of the adsorption-induced PL changes was compared for the different atmospheres. Reversibility by the simple cycle high vacuum pumping (HV) -adsorption-HV could be reached only for C2H5OH. The PL changes induced by H2O and C-2(CN)(4) adsorption were good (H2O) or partially (C-2(CN)(4)) reversible in combination with heat treatment at 420 K in HV. The O-2-adsorption-induced PL changes are not reversible by simple or modified cycles.