Abstract :  Expanded graphite (EG) and boron nitride (BN) were used as fillers to impart thermal conductivity (TC) while maintaining electrical insulation of a homopolymerized cycloaliphatic epoxy matrix. Even though EG leads to a higher increase of TC than BN (550% of enhancement with only a 7.5 wt.% of EG), EG is also electroconductive and its ratio in the formulation must be lower than the percolation threshold. Formulations with proportions between 2.5–7.5 wt.% of EG as the filler and mixtures with EG and a 40 wt.% of BN were thermally polymerized and composites with 70 wt.% of BN and 2.5/5.0 wt.% of EG were also prepared under pressure and then cured in the oven. Over 2 W/m K was achieved (i.e. more than 1500% of enhancement in reference to the neat epoxy). The composites containing a 40 wt. % of BN and 2.5 wt % of EG or 70% wt. % of BN and 5 wt % of EG were found to keep the insulation character. Mechanical and thermal characteristics of the prepared materials were also evaluated.