Light scattering from porous silicon Layers has been investigated. It reveals that scattering depends strongly, first, on the layer thickness and secondly on the doping level of the substrate. During the formation of porous silicon (PS) on p-type Si material, the bottom of the PS layer develops a roughness which is responsible for the observed scattered light. The two other possible contributions to the scattering, the bulk PS and the interface between PS and air were found to be negligible (< 1%) compared with the first one and independent of the layer thickness. Mechanical roughness and optical scattering measurements give a bottom interface flatness of the order of 100 Angstrom per micron of layer and a mean lateral spatial frequency of the oscillations of 1 mu m(-1), independent of the sample thickness. In contrast the back interface of layers from a heavily doped p + substrate remains almost hat and the level of scattering is low.