Polypropylene (PP) films with high beta-crystal content were drawn biaxially under various conditions to obtain highly porous films. Their pore size distribution and pore volume, measured by the mercury intrusion method, were found to depend on the draw ratio. Wide-angle X-ray diffraction of the drawn films in edge view showed that the alpha and beta crystals are oriented in directions parallel and perpendicular to the drawing directions, respectively. The morphologies of the films drawn to various draw ratios could be explained by a two-step mechanism that involves the deformation of lamellar blocks and the formation of microfibrils. The oxygen and carbon dioxide permeabilities of these films were also measured and compared with those of films drawn uniaxially at constant width. The high permeability coefficients (of the order of 10(5) Barrer) suggested that the biaxially drawn films can be used as gas exchange membranes.