ZSM-5-type zeolite membranes were synthesized at 130 degrees C on alumina supports coated with seed crystals by vacuum seeding. Compact and uniform ZSM-5 layers were formed on the support surfaces when the amount of seed was less than 1 mg/cm(2). Higher loading of seed crystals resulted in an asymmetric structure with a compact and uniform ZSM-5 layer on top, the support at the bottom, and the seed layer in between. Fifty-one membranes were prepared on supports coated with different amounts of seed. About 70% of the membranes synthesized on seeded supports were impermeable to nitrogen before calcination, whereas all nine membranes synthesized on plain supports were permeable to nitrogen before calcination. One-third of the membranes synthesized on seeded supports had N-2/SF6 ideal selectivity higher than the quality criterion of 40 after calcination. Membranes with high N-2/SF6 ideal selectivity also showed high n-C4H10/i-C4H10 separation selectivity.