Heat treatment of polyacrylonitrile (PAN) leads to products with semiconductor to metal-like conductivities. The electrical properties of these-materials are further modified by ion implantation. It is noted that the conductivity (10−7(Ω cm)−1 of heat treated PAN at 435 °C (PAN435) increases upon ion implantation with arsenic reaching a maximum value of 1.4 × 10−1 (Ω cm)−1 at a dose of 5 × 1016 ion/cm2 and energy of 200 KeV. On the other hand, ion implantation of the more conducting heat treated PAN at 750 °C (PAN750) leads to a decrease in the electrical conductivity of the material. The data also indicate that the decrease in conductivity observed after implantation of PAN750 is due to the breakup of the graphite-like extended electronic states. The temperature dependence of conductivity provided further information on the structures of the pyrolyzed and of the ion implanted samples. Whereas the conductivity-temperature behavior of PAN750 is consistent with the Cohen-Jortner model for transport in inhomogeneous and disordered solids, the ion implanted sample (PAN750I) displays a behavior which can be described most adequately in terms of a variable range hopping conduction mechanism.