AbstractThe representation and interpretation of dynamic mechanical properties of heterogeneous polymer–polymer composites are discussed in terms of equivalent mechanical models and the viscoelastic form of the well‐known Kerner equation. Model parameters calculated from dispersed phase volume fraction and matrix Poisson's ratio (using the Kerner equation) are in fairly good agreement with experimental values for systems comprising soft inclusions in a hard matrix. The effects of partial phase inversion on dynamic properties are discussed in terms of an extension of the Kerner equation. Model calculations indicate that the in‐phase component of the complex modulus depends primarily on dispersed phase volume concentration, while the out‐of‐phase component depends on both the concentration and the morphology of the dispersed phase. Although substantial information about the microstructure of polymer–polymer composites can in principle be deduced from dynamic measurements, quantitative correlation between dynamic properties and use properties such as impact strength (which may have a quite different dependence on structural parameters) is probably