The effect of hydration and dehydration on the elec. field gradient at the 27Al nucleus in ZSM-5 zeolites was studied by magic angle spinning NMR at 3 different magnetic field strengths. Dehydration causes an increase of the 27Al NMR line width due to an increased quadrupole interaction. The dehydration causes a redistribution of elec. charges and/or a conformation change of the Al-O tetrahedra. The latter possibility seems to be most important since the effect is cation dependent, as shown by results on Na-ZSM-5 and H-ZSM-5. Similar effects are obsd. for faujasite H-Y and amorphous SiO2/Al2O3.