At the exit surface of a photonic crystal, the intensity of the diffracted wave can be periodically modulated, showing a maximum in the ???positive??? (forward diffracted) or in the ???negative??? (diffracted) direction, depending on the slab thickness. This thickness dependence is a direct result of the so-called Pendell¨osung phenomenon, consisting of the periodic exchange inside the crystal of the energy between direct and diffracted beams. We report the experimental observation of this effect in the microwave region at about 14GHz by irradiating 2D photonic crystal slabs of different thickness and detecting the intensity distribution of the electromagnetic field at the exit surface and inside the crystal itself.