The structural and optical properties of low and high porosity luminescent porous silicon layers have been studied. It has been shown that the luminescence intensity is strongly dependent on the porosity and not on the surface area. Double-crystal X-ray and electron diffraction patterns indicate that freshly anodized luminescent porous silicon is crystalline. Both techniques show an increasing distortion of the lattice planes with increasing porosity, implying that the distortion is strongly linked to the smallness of the crystallites constituting the layers.