In this study, Al-MFI and B-MFI (Si/B=100 and Si/B=50) hollow fibres with nanocomposite architecture have been prepared on α-alumina by pore-plugging hydrothermal synthesis at 443K for 89h using a precursor clear solution with molar composition 1 SiO 2 : 0.45 TPAOH: 27.8 H 2 O: xH 3 BO 3 (x=0–0.02). The synthesized materials were characterized by XRD, SEM, ICP-AES and 29 Si, 27 Al- and 11 B-MAS-NMR, revealing the genesis of well-intergrown materials with isomorphously substituted boron in the MFI unit cell. The pure ethylbenzene permeance within these membranes decreased in the order Al-MFI>B-MFI (Si/B=50)>B-MFI (Si/B=100). All the MFI materials were selective to p-xylene in the vapour permeation of ternary p-/m-/o-xylene mixtures and quaternary p-/m-/o-xylene/ethylbenzene model mixtures in the temperature range 400–650K and for total feed vapour pressures lower than 4.5kPa. The p-/m-xylene separation factor increased in the sense Al-MFI<B-MFI (Si/B=100)≈B-MFI (Si/B=50). The p-/m-xylene and p-/o-xylene separation factors, as well as p-xylene mixture fluxes, decreased with the ethylbenzene feed concentration, probably due to configurational entropy effects in the mixture adsorption patterns. The membranes showed however slightly preferential p-xylene permeation than ethylbenzene at low total aromatic vapour pressures despite the similar kinetic diameter of both molecules (5.8 vs. 6.0Å), the B-MFI (Si/B=50) material achieving a p-xylene/ethylbenzene separation factor as high as 5.