Effective separation of xylene isomers is possible with pervaporation through MFI-silicalite membranes. The effect of microstructural differences in silicalite membranes on the separation of xylene isomers was investigated. MFI-type zeolite membranes with random and ordered orientations were synthesized using a seeded growth method with and without organic templates. It was found that template-free random oriented membranes were capable of pure component p/o xylene separation factors of about 20 and p-xylene flux of 0.3 kg m-2 hr-1 . The membranes with preferred orientation showed comparable flux of xylene isomers but decreased selectivities ranging from 2 to 3. These results indicate that microstructure strongly affects the ability of the membrane to perform xylene separation.