Five different polypropylene resins were selected to develop microporous membranes through melt extrusion and stretching (cast film process). The effect of the polymer melt elongation properties on the row-nucleated lamellar crystallization was investigated. The arrangement and orientation of the crystalline and amorphous phases were examined by WAXD (wide angle X-ray diffraction) and FTIR (Fourier transform infrared) methods. The extrusion and cooling parameters were adjusted properly to obtain uniform precursor films with appropriate morphology. Annealing, cold and hot stretching were consequently employed to generate and enlarge the pores. It was found that molecular weight was the most important material factor that controlled the membrane structure. The role of annealing and stretching parameters was also investigated. The permeability to water vapor (under atmospheric condition) and nitrogen (under pressure) was measured. It was observed that the permeability increased with increasing pressure and went through a transition from Knudsen diffusion to Poiseuille flow. (c) 2007 Elsevier B.V. All rights reserved.