The dielectric properties of epoxy-based nanocomposites containing core-shell nanoparticles in this case TiO ₂ /SiO ₂ and Al ₂ O ₃ /SiO ₂ are investigated to examine the effect of the interphase layer. Core-shell nanoparticle were synthesised and epoxy nanocomposites were prepared for characterisation by dielectric spectroscopy (0.1 Hz - 1 MHz) and the glass transition temperature measured. The results were analysed demonstrating dielectric properties which appear counter to some generally assumed hypotheses. For example: Unlike non-core-shell nanoparticles in case of Al2O3/SiO2 core-shell nanoparticles for 1 wt. % a significant drop in the real permittivity is reported. This is attributed to the hypothesised changes in the particle interphase layer. It is also reported that as the filler concentration increased the glass transition temperature reduces. As the glass transition temperature is mainly dependent on the polymer matrix it is postulated that this could be a result of interference of the core-shell nanoparticle with the mobility dynamics and changes in the free volume. The effect of interactions between the core and shell of the nanoparticle and changes in the concentration of localised energy states are also discussed in the paper.