This study proposes a new model for describing the electrical behaviour of nanocomposites. Unlike other models in the literature, this model has concentrated on the role of an interphase layer within the boundaries of nanoparticles. The experimental part investigates this role by filling an epoxy matrix with two types of surface-modified silicon nitride nanofiller: (a) the particles were dried at 200 °C, and (b) the particles were calcinated at 1050 °C. Electrical characterization showed that the epoxy which was filled with the calcinated particles has considerably better dielectric performance. Given that thermal and dielectric spectroscopy results demonstrate that the matrix molecular dynamics and polar content are comparable for all the investigated samples, the variations in the dielectric performance point to the particle interphase as an essential reason. As shown by infrared spectroscopy, the complex surface chemistry of the dried particles suggests a particle interphase with a high concentration of localized electronic states, which may enhance charge transport through hopping/tunnelling conduction. On the other hand, calcinating the particles results in a particle interphase with wider band gap, which may work as an energy barrier for charge movement. Consequently, this study highlights the paramount importance of particle interphase for designing dielectric properties of nanodielectrics.