In this work, several beta-zeolite-incorporated polymer composite membranes were fabricated with the solution-casting, method. The zeolite loadings were 10, 20, and 30 wt % respectively. Scanning electron microscopy characterization showed that the zeolite particles could be uniformly distributed in the whole polymer matrix. Gas permeation results demonstrated that after the incorporation of the same beta-zeolite, the polyimide exhibited a significant increase in gas Permeability but a decrease in permselectivity, and both were quite pronounced at high zeolite loadings; this resulted from the loose structure that formed. The poly(ether sulfone) composite membranes showed obvious increases in both permeability and selectivity, and the permeability increase Was considerably greater at higher zeolite loadings. The permselectivity increase could possibly be attributed to the pore sieving and preferential adsorption of beta-zeolite entities for the test gases as the heat treatment may have resulted in the formation of a defect-free microstructure. However, breakthrough of the upper-bound line was not achieved for these composite membranes, as reflected by Robeson plots. Our results suggest that changes in membrane performance not only are attributable to the properties and content of beta-zeolite particles but also depend on the heterogeneous microstructure created by zeolite entities, (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 112: 9-18, 2009