Electron paramagnetic resonance (EPR) and photoluminescence (PL) techniques were employed to investigate the influence of molecules adsorption (H2O, O-2, C2H5OH, C-2(CN)(4)) on properties of porous silicon (per-Si). A photoinduced decrease (H2O atmosphere) and increase (O-2 atmosphere) of the silicon dangling bond concentration N-DB were detected while PL enhancement (H2O) and quenching (O-2) took place. Decrease of N-DB and quenching of the PL was observed under C2H5OH adsorption. Adsorption of C-2(CN)(4) did not change the EPR signal of the DB. A new EPR signal of [C-2(CN)(4)](-) radicals appeared and the PL intensity decreased dramatically at the same time. The results are discussed on the basis of three main mechanisms of the influence of molecules on the per-Si PL properties : (i) chemical and photochemical modification of recombination centers (H2O and O-2); (ii) dielectric exciton quenching (C2H5OH); and (iii) destruction of excitons by strong Coulomb fields of the adsorbed molecules (C-2(CN)(4)).