In a fusion reactor, tritium will be continuously produced in the breeding blanket and a dedicated tritium extraction and removal system (TERS) is used to recover the newly bred tritium. In the TERS for the solid blanket, zeolite membranes are proposed to be used to pre-concentrate HT/H-2 in He before Pd/Ag membranes. Therefore, permeation experiments were carried out using two experimental facilities to determine the permeances of all hydrogen isotopologues (Q(2) = H-2, HD, D-2, HT, DT, T-2) through MFI-ZSM-5 membranes. This study aimed at investigating the existence of isotopic effects on the Q(2) permeation and hence on the separation of Q(2) from He. The results exhibit a strong dependency of the permeance with the mass of the isotopologue: the selectivity decreases from around 1.8 for H-2/He to 1.3 for D-2/He down to 1.1 for T-2/He. This phenomenon is attributed to the existence of defects in the zeolite structure through which the molecules permeate according to Knudsen diffusion. These results were observed for inactive and active Q(2) mixtures and no other effects related to the dipole moment of the asymmetric isotopologues (HD, HT, DT) or tritium decay were observed. Moreover, these results were obtained for three MFI-ZSM-5 membranes, showing a high reproducibility. Our results demonstrate that the pre-concentration of Q(2) in He with MFI-ZSM-5 membranes appears to be challenging in the scope of the TERS. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.