Oriented MFI membranes made by secondary growth on the surface of alpha -Al2O3 support disks have been evaluated for the separation of xylene isomers vapors in the temperature range 22-275 degreesC and feed partial pressures up to similar to0.7-0.9 kPa. It is found that the separation performance of these membranes is directly related to the synthesis conditions and the resulting membrane microstructure. Thick (12-18 mum) and c-oriented membranes (type A membranes) made by 24-h growth at 175 degreesC exhibit single-component p-xylene/o-xylene permselectivity as high as 150 at 100 degreesC but very modest separation factors (usually <5) are obtained with binary feed mixtures, a result of a drastic increase in the flux of o-xylene in the presence of p-xylene. Thin (2-3 m) and (h0h)-oriented membranes (type B membranes) made by 120-h growth at 90 degreesC exhibit comparable single-component and binary permeation behavior but suffer from lower selectivities/separation factors (up to 12) as a result of cracks formed after calcination for template removal. It is found that the addition of n-hexane in a binary p-xylenelo-xylene feed in the case of type A membranes or the dip-coating of type B membranes with a surfactant-templated silica sol results in a considerable improvement of mixture separation factors up to 60 or 30-300, respectively. This improvement is attributed to preferrential adsorption of n-hexane in nonzeolitic micropores/grain boundaries in the case of type A membranes or to selective sealing of cracks by mesostructured silica in the case of type B membranes. Both type A and type B membranes exhibit, at 100-125 degreesC, p-xylene permeance [(2-5) x 10(-8) mol m(-2) s(-1) Pa-1] and p-xylene/oxylene separation factors (60-300) that are comparable to or higher than the values reported for other MFI membranes in the literature.