ZSM-5, a prominent member of the new generation of high silica zeolites, is a potential catalyst. The activity of ZSM-5 type synthetic zeolites with respect to acid-catalysed reactions is a linear function of the Al content1 which can assume extremely low values, but in catalytic applications is typically 1–3 Al per unit cell containing 96 Si+Al atoms. We now present substantial evidence that the Al distribution in ZSM-5 crystals is non-uniform. We first noted zoning of Al in ZSM-5 when examining scanning X-ray micrographs (Al Kα) of 50 µm crystals. The comparatively large crystals necessary for detailed measurements of Al concentration profiles using electron probe analysis suggest that Al is invariably concentrated in the rim portion of the crystals. No comparable zoning has been reported previously for zeolites.