The authors report on the first experimental observation of transient laser-induced gratings in self-supporting porous silicon membranes with different porosities. The observed grating decay in time is very fast (hundreds of picoseconds) at room temperature and shortens both with decreasing porosity and grating period. A preliminary microscopic model for this behaviour is suggested, The third-order non-linear optical susceptibility chi((3)) (532 nm) in porous Si was found to be chi((3)) approximate to 7 x 10(-11) e.s.u.