The influence of the dispersion of vapor grown carbon nanofibers (VGCNF) on the electrical properties of VGCNF/epoxy composites has been studied. A homogeneous dispersion of the VGCNF does not imply better electrical properties. The presence of well distributed clusters appears to be a key factor for increasing composite conductivity. It is also shown that the main conduction mechanism has an ionic nature for concentrations below the percolation threshold, while above the percolation threshold it is dominated by hopping between the fillers. Finally, using the granular system theory it is possible to explain the origin of conduction at low temperatures. ; We acknowledge the Foundation for Science and Technology,Lisbon, for the financial support through the 3 Quadro Comunitário de Apoio, the POCTI and FEDER programs, projects PTDC/CTM/69316/2006, NANO/NMed-SD/0156/2007, and PEst-C/CTM/LA0025/2011. We also acknowledge the FCT grants SFRH/ BD/60623/2009 (J.S.) and SFRH/BD/41191/2007 (P.C.), the Joint Luso-American Foundation (FLAD)–NSF U.S. Research Networks Program research grant (F.W.J.v.H. and D.K.). We also thank Albemarle for the hardener, Hexion Specialty Chemicals for the epoxy resin and Applied Sciences for providing their facilities. We also thank to J. Oliveira for the collaboration.