Material response under uniaxial stress has been reported for heterogeneous systems : low density poly(ethylene)-poly(styrene). The systems differed in a component ratio and in a content of the macromolecular modifier type compatibilizer. Tensile properties have been determined for the systems in a molten state (473 K) and below T-g as a function of elongation rate. The modifier which diminishes the interfacial tension is advantageous for mechanical properties of the system-both in the melt and the solid state. The differences in mechanical characteristics have been related to a specific phase structure of polymer systems.