The performances of various zeolite filled polymeric membranes in the separation of n-pentane from i-pentane were investigated as a function of zeolite loading and various experimental conditions. Polydimethylsiloxane (PDMS) was chosen as the polymer phase and HZSM-5, NaZSM-5,4A and 5A were used as zeolite fillers. Different Si/Al ratios and different activation temperatures were tested for ZSM-5 and A type zeolites, respectively. No improvement with respect to the n-pentane/i-pentane ideal selectivity of the original polymeric membrane could be obtained for the mixed matrix membranes investigated in this study. Interactions occurring in the zeolite-polymer interface seem to play a significant role in the results obtained. n-Pentane consistently showed lower permeability than for the pure polymer. On the other hand, i-pentane permeability was not reduced and was even increased with increased zeolite loading in some cases. It is hypothesized that these trends reflect differences in interaction between the two chemically similar penetrants with the "interphase" region connecting the bulk polymer and dispersed zeolite phases. The Si/Al ratio, the cation type and the activation temperature of the zeolite employed also seem to affect the performance of the zeolite-PDMS mixed matrix membranes. (C) 2001 Elsevier Science B.V. All rights reserved.