The effect of zeolite particle size on the performance of silicalite-PDMS mixed matrix membranes is investigated at two different zeolite loadings. The separation properties of the membranes prepared are characterized by permeability measurements for O-2, N-2 and CO2 gases. The permeabilities of the silicalite-PDMS mixed matrix membranes are determined to increase with increasing particle size. The variations occurring in the permeability values with changes made in the particle size are much more pronounced at the higher zeolite loading. The ideal selectivity values corresponding to the mixed matrix membranes, on the other hand, generally seem to be less affected by the changes made in the particle size. The permeability values corresponding to the mixed matrix membranes exceed those pertaining to the original polymer membrane only at relatively higher zeolite loadings and/or for relatively larger particle sizes. The variations occurring in the permeabilities and selectivities with changes made in the zeolite particle size may be responsible for the different values of these parameters reported in the literature for the same types of zeolite filled polymeric membranes. (C) 2000 Elsevier Science B.V. All rights reserved.