Light-emitting porous silicon (PS) layers prepared by anodic oxidation on p-substrates have been studied by micro-Raman scattering and microphotoluminescence spectroscopy simultaneously. A complementary study by transmission electron microscopy has been made. A change in microscopic structure with depth is observed as well as a red shift of the luminescence as the focalized laser beam approaches the bulk substrate. This result pointed out a correlation between the structural depth inhomogeneity and optical properties of porous silicon in agreement with quantum size effects.