Porous silicon membranes having 30 mum thickness, 1-2.5 mm side and nominal porosity in the range from 55 to 65% were formed in square localized areas of p(+) silicon and were investigated by multicrystal X-ray diffractometry, X-ray absorption, and X-ray synchrotron section topography. Preliminary results of this study show that the structural characteristics of the membranes are almost the same as those of the porous silicon formed on bulk substrate. From the mechanical point of view, the lack of substrate constraint induces membrane bending, whose amount and irregularity increase with the membrane size. However, the mechanical deformations in porous membranes up to sides of 1-1.5 ram are limited enough to allow manufacturing of reliable solid-state sensor devices, as reported in recent literature.