MFI-type zeolite membranes were hydrothermally synthesized as a thin layer on macroporous -alumina discs coated with 1m MFI-type seed crystals as seed. The effect of soda (Na2O) concentration on the morphology of the membranes was investigated. Soda concentration, and thus the hydroxyl ion concentration, was changed between 0.25 and 6.5 mol/mol gel. At low soda concentrations the crystals forming the layer exhibited mostly (h0h)/c-axis orientation, but at high soda concentrations a membrane layer formed from randomly oriented crystals. The membranes showed high H-2/n-butane ideal selectivity of 476 and 36 at 25 degrees C and 150 degrees C, respectively.