Zeolite-polymer mixed matrix membranes were prepared by filling polydimethylsiloxane (PDMS) with clinoptilolite in various cationic forms of K, Mg, H, and Na. The membranes were characterized by scanning electron microscopy as well as measurements of O-2, N-2 and CO2 gas permeabilities. The results indicated the presence of an optimum zeolite loading in the mixed matrix membranes maximizing permeability. The type of the cationic form of clinoptilolite used affected the permeabilities and ideal selectivities of the zeolite-polymer mixed matrix membranes investigated to some degree and some improvements were obtained when compared to the ideal selectivities of the original polymeric membranes.