Dielectric measurements have been performed on neat acrylic core-shell particles, and core-shell modified-epoxy networks in order to assess the accuracy of the Maxwell-Wagner-Sillars (MWS) model in describing the dielectric properties of heterogeneous polymeric systems. Measured and predicted interfacial polarizations are compared. The temperature dependence of the frequency of maximum interfacial dielectric loss process is well described by the MWS model and from the temperature dependence of the conductivity of the two phases. However, the measured and predicted absolute values differ and a distribution in the interfacial relaxation time is observed. This is attributed to the effect of a gradient in concentration across the interface between the acrylic particles and the epoxy matrix. It is thus concluded that the nature and properties of interphases can strongly influence the dielectrical properties and interfacial polarization processes in polymer blends, and that theoretical models which do not take into account details of interfacial charge transfer might not be reliable in describing the morphology of heterogeneous polymeric systems.