Novel interpenetrating polymer networks of polypropylene/poly(n-butyl acrylate) have been prepared via in-situ polymerization within a biaxially stretched microporous polypropylene film. The volume fraction of poly(n-butyl acrylate) in composite films was higher than 40%. The fine-dispersed anisotropic dual-phase continuous interpenetrating network structure was confirmed by surface analysis and transmission electron microscope observation. The interfacial action between polypropylene and poly(n-butyl acrylate) was relatively clear. The elastic moduli of the composite films were lower than that of the matrix film. The swelling of poly(n-butyl acrylate) in organic solvent was strikingly limited by the polypropylene network. All these properties were in agreement with the anisotropic interpenetrating network structure. (C) 1997 Elsevier Science Ltd.