Porous polymer films were generated by biaxial stretching of polypropylene (PP) and high-density polyethylene (HDPE) filled with various amounts of calcium carbonate particles. The porosity of the films was measured by mercury porosimetry, and the obtained results were related to the processing conditions and to the morphology development during the biaxial stretching. The results showed that increasing the calcium carbonate (CaCO3) concentration and the draw ratio resulted in porosity increase for PP-based composite films and in a decreased porosity in HDPE-based composite films. Such peculiar behavior was connected to interfacial specific interactions between the matrix and the dispersed particles as well as to the crystallinity of the films. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 3425-3436, 2012