X-ray diffraction and reflectometry allows the measurements of various parameters (thickness, porosity, roughness and strain) of thin layers of porous silicon. Measurements on n-type porous silicon layers of different doping give very different results: for lightly doped samples, the layer properties vary smoothly as a function of formation time, while for heavily doped samples several regimes are observed for short formation times. X-ray satellites have been observed in the X-ray reflexion or diffraction from holographic gratings.