The electrical and thermal behavior of epoxy composites reinforced with different contents of multi-walled carbon nanotubes (from 0.1 to 0.4 wt% CNT) is studied when they are subjected to relatively high DC voltages (from 1 to 100 V). These materials obey Ohm's law, reaching values of electrical conductivity in the range of 0.01-0.5 S/m. The transported electric current leads to a significant increase of temperature, which is a result of the Joule heating effect. The temperature increases to 40 degrees C in CNT/epoxy composites when applying 100 V. The study of heating due to Joule's effect gives information about the electrical transport mechanisms implied. It is also confirmed that both, electrical conductivity and Joule's heating effect depend on the morphological features of the composites. The functionalization of CNTs decreases the electrical conductivity of composites but increases their corresponding Joule heating, due to the strong interface between the nanotubes and matrix, which hinders the formation of pathways in CNT in direct contact. The technique of CNT dispersion applied also affects to the increase of temperature induced by the electrical current. (C) 2013 Society of Plastics Engineers