Emission and excitation photoluminescence spectra of porous silicon thin layers have been investigated at natural oxidation. The shift of both types of spectra to high-energy region with time has been shown. Analysis of excitation spectra points out the indirect behavior of electron transitions responsible for visible photoluminescence, which remains unaltered at natural oxidation. The value of optical bandgap is estimated in each case. It is shown that the optical bandgap broadens during oxidation due to size reduction of silicon nanocrystallites. ► Porous silicon emission and excitation spectra blue shifted at natural oxidation. ► Excitation spectra points out the indirect behavior of electron transitions. ► Optical bandgap broadens during oxidation due to Si nanoparticles size reduction.