Zeolite ZSM-5 layers (up to ca. 30% by weight) have been synthesized on cordierite substrates, following either a direct hydrothermal synthesis procedure or a secondary growth method, in this case after seeding of the support. The Si/Al ratio in the synthesis gel ranged from 14 to 100, but layers with a high Al content (i.e. a low SUM ratio) could not be prepared directly on the cordierite support. However, MFI layers with a low Si/Al ratio were readily grown after depositing an intermediate Si-rich layer. The results also show that the Si/Al ratio of the synthesis gel has a direct effect on the morphology, crystallinity and orientation of the MFI layer formed. (C) 2003 Elsevier B.V. All rights reserved.