The viscoelastic properties of binary thermoset and thermoplastic polymer blends were investigated in connection with blend morphologies. Christensen and Lo＇s model was used to predict mechanical coupling effects in such binary multiphased systems by accounting for the actual morphology of samples. Thus, it was shown that the magnitude of mechanical coupling effects between phases in polymer blends, as in composite materials, depends not only on mechanical properties and relative content of each phase but also on the geometric arrangement of the polymeric phases. Furthermore, based on both theory and experiment, a well-suited probe of blend morphology was also proposed. (C) 2000 John Wiley & Sons, Inc.