In this research, the preparation and characterization of polypropylene (PP) and natural zeolite composites were studied. Natural zeolite mined in Gordes, Turkey was used as an alternative filler to CaCO3. Films were prepared by the extrusion of PP, and surface-modified zeolite was made by polyethylene glycol 4000 with 2-4% zeolite. Zeolite-filled composites had densities between 0.73 and 0.83 g/cm(3) and had void fractions of 0.07-0.20. Although the permeability of water vapor through 2% zeolite-filled composites was very small, 4% zeolite-filled films had very high permeabilities. The yield stresses of 2-4% zeolite-containing films were around 26-27 N/mm(2) and were lower than that of PP, which indicated no adhesion between PP and zeolite. The effect of zeolite on the thermal degradation behavior in air and in a N-2 atmosphere was also studied. In air, zeolites did not cause the oxidation of PP. In a N-2 atmosphere, although the start of the thermal degradation of PP was retarded by zeolite, composites degraded at a faster rate than PP once the degradation started. At a processing temperature of 200 degreesC, zeolites had no effect on the degradation of PP.