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Stress relaxation and dynamic mechanical measurements were carried out for two types of heterophase polyblends. One is obtained by blending homopolymers of butadiene and styrene (high impact polystyrene of HIPS); the other by blending homopolymer of butadiene with a triblock copolymer of styrene-butadiene-styrene (SBS/B). It was found that for HIPS, time temperature superposition is difficult, and the shift factors cannot be adequately interpreted by a reasonable model. For SBS/B it is impossible to carry out superposition. Possible mechanisms giving rise to the new transition are discussed. (Author)