AbstractThe physical properties of heterogeneous polymer–polymer composites such as rubber‐modified plastics depend not only on the properties of the constituent polymers, but also on structural characteristics such as phase geometry and morphology. The dependence of the viscoelastic properties of particulate composites on phase morphology and composition has been treated in a previous paper. This analysis is further modified and applied to dynamic Young's modulus (E*) data on several types of heterogeneous acrylic polyblends prepared in this laboratory. By taking into account interparticle interactions, copolymerization effects, and phase inversion, a nearly quantitative representation ofE*is obtained. Deviations from calculated behavior can be explained by assuming more complicated models of phase structure and composition and by postulating higher‐order and temperature‐dependent interaction