Blends of polypropylene (PP), the triblock copolymer polystyrene-block-poly(ethylene-stat-butylene)-block-polystyrene (SEBS), and processing oil were found to form thermoplastic interpenetrating polymer network (IPN) structures in a composition range from about 10 to 55% by weight polypropylene. The IPN structure was confirmed by electron microscopy and by solvent extraction. At high elongations, the cocontinuous blends showed a stress-strain behavior similar to rubber and no signs of the typical necking phenomenon normally associated with polypropylene at large deformations. The processing oil used to improve the processing properties of SEBS was found to partly dissolve in the polypropylene phase, causing a marked lowering of the polypropylene glass-transition temperature. The distribution coef ficient for oil between polypropylene and SEBS was estimated to be 0.35. While the degree of crystallinity of polypropylene did not vary with blend composition, the melting temperature decreased from 162.7 degrees C in the pure polypropylene to 149.3 degrees C in the blend with lowest polypropylene content. The large melting point depression suggests that polypropylene, the EB fraction of SEBS, and the oil may form a homogeneous melt phase. This probably explains the formation of an IPN structure on cooling.