Microporous polymer membranes were fabricated by extrusion, thermal annealing, and subsequent biaxial stretching of beta nucleated isotactic polypropylene. Fabrication conditions were optimized based on prior studies so that the influence of annealing on the precursor film crystallinity and the pore formation in such films upon stretching could be investigated. Annealing perfected the crystalline structure of these materials, as evidenced by thermal and x-ray techniques. A mechanism detailing the annealing and pore formation process during biaxial stretching is presented. An appropriate metric was developed to relate the crystallinity of the precursor films to the porosity of the films after biaxially stretching. The porosity developed in biaxially stretched films increased substantially when its precursor was annealed near the melting temperature of beta crystalline material, enabling the production of high permeability microporous membranes. (C) 2013 Elsevier Ltd. All rights reserved.