Free-standing microfabricated (MFI) zeolite micromembranes were successfully fabricated on a silicon substrate. Three zeolites including Sil-1, ZSM-5 (Si/Al = 60) and ZSM-5 (Si/Al = 40) were prepared. The zeolite micromembrane units were tested for gas permeation and separation. The micromembranes displayed an order of magnitude higher permeability (i.e., H-2 and He) compared to traditional supported zeolite membranes and possessed excellent permselectivity. Separation was conducted for a gas mixture containing hydrogen, methane and carbon dioxide. Zeolite chemistry plays a role in membrane separation. Sil-1 micromembrane selectively permeated the heavier hydrocarbon and carbon dioxide resulting in a hydrogen-enriched retentate stream, while hydrogen was concentrated at the permeate outlet of the ZSM-5 micromembrane. (C) 2004 Elsevier Ltd. All rights reserved.