A novel microcellular porous structure, the product of a small proportion of a semicrystalline thermoplastic elastomer and a large proportion of low molecular weight oil, was examined. The system exhibited a unique three-dimensional continuous polymer network, consisting of interconnected spherical cells of a few tens of micrometers in diameter. In this report, details of morphology and stress-strain relationship of the system, which is made from hydrogenated poly[butadiene-b-(styrene-random-butadiene)] (H-SBR) as a semicrystalline thermoplastic elastomer and straight asphalt as an oil, were investigated and analyzed. The system exhibited inverse S-shape type stress-strain curves, but was characterized by extension rate and polymer concentration dependency. It was demonstrated that the stress-strain curves of the system can be described by a simple combination of that of two phases: asphalt-rich phase and H-SBR-rich phase. Calculations of a simple parallel model quantitatively agreed with experimental data, especially in the case of higher polymer concentrations (about 12 to 20 vol%.) where uniform polymer network is formed over the whole system. Disagreement between calculation and experimental data was, however, found in the cases of very small proportions of H-SBR (< similar to 8 vol %), where the polymer network structure in the system was imperfect. (C) 2004 Wiley Periodicals, Inc.