Polyethersulfone (PES)-based mixed matrix membranes (MMMs) with the incorporation of inorganic fillers of different shapes (lamellar Na-montmorillonite (MMT) clays and spherical TiO2 nanoparticles) were prepared in this study, and the resulting MMMs were characterized by TGA, DSC, XRD, SEM, and TEM. It was found that inorganic filler agglomeration became more serious at higher-filler-content MMMs. In the case of PES/MMT MMMs, both the CO2 and CH4 permeabilities increased significantly with the increasing filler content and consequently the gas selectivity was greatly reduced. At high MMT loadings ( >= 10 wt.%), Knudsen diffusion became the predominant gas transport mechanism. A different trend was achieved in the case of PES/Ti0(2) MMMs. The CO2/CH4 selectivity increased from 24.5 (pure PES membrane) to a maximum value of 38.5 at 4 wt.% TiO2 MMM and then decreased with a further increase in TiO2 content (e.g. 17.3 for 20 wt.%). The formation of interface voids and membrane defects in MMMs contributed to the high gas permeabilities and low gas selectivity. (C) 2012 Elsevier B.V. All rights reserved.