Microporous polymer membranes were produced from beta nucleated isotactic polypropylene using a solvent-free process involving extrusion and biaxial stretching. Pore formation, previously shown to depend upon the quantity of perfected beta crystalline material in the film, was found to also depend upon stretching temperature. The pore structure of membranes produced under a variety of processing conditions was investigated by SEM, flow porometry, and gas permeation techniques. The mechanism of gas transport through the membrane was determined using permeability experiments and the film pore size distribution. Gas permeability was correlated with film porosity and compared to the permeability of an array of porous polypropylene films. High quality microporous membranes were produced without the use of the strict process control required for traditional extruded films, and such membranes exhibit performance comparable to commercially available Celgard films. This process flexibility provides an avenue toward coextruded composite membranes for a range of high-value applications. (C) 2013 Elsevier Ltd. All rights reserved.