The gas permeation and separation performance of polycrystalline MR-type zeolite membranes is strongly dependent on the number and type of intercrystalline pores in its structure. Herein we show that the role of such domains is affected by how a membrane is pre-treated before use to remove adsorbed species (e.g. moisture and organics). This 'pre-treatment' step appears to be crucial not only to obtain reliable permeation data, but also to improve the membrane separation performance in practical applications. We illustrate this idea by using a collection of tubular nanocomposite MFI-alumina membranes showing different quality for the separation of n-butane/H-2 mixtures and submitted to different pretreatment protocols. The influence of each protocol on the final separation performance of the membranes depends on their quality, namely on the density of intercrystalline defects or non-zeolite pores in their structure. Moreover the quality of the support affects the final membrane performance. (C) 2008 Elsevier B.V. All rights reserved.