MFI type zeolite membranes were prepared on seeded tubular alumina supports in a flow system where the synthesis solution was circulated through the support with flow rates of 6, 24 and 48 mL/min. Syntheses were carried out using clear solutions, at 80 degrees C and 95 degrees C at atmospheric pressure in the flow system, and at autogeneous pressure in the batch system. The membranes synthesized in the flow system had thickness of 1-2 mu m, and were thinner than the membranes synthesized in the batch system. After one synthesis step, the N-2/SF6 ideal selectivities at room temperature and n-C4H10/i-C4H10 (50-50%) separation selectivities at 200 degrees C of the membranes synthesized in the flow system were between 9.1-14.9 and 6.1-7.6, respectively, and those of the membrane synthesized in the batch system were 27 and 6.1. Multiple synthesis steps improved the membrane quality so that the membrane prepared with two consecutive steps had N-2/SF6 ideal selectivity of 141 and n-C4H10/i-C4H10 separation selectivity of 44 at 200 degrees C. Among many zeolite membranes reported in literature, these membranes are one of the few zeolite membranes synthesized in a flow system and the first MFI type membranes synthesized in a flow system with circulation of the synthesis solution. (c) 2006 Elsevier Inc. All rights reserved.