The electrical properties and electromagnetic shielding effectiveness (EM SE) of nanocomposites consisting of heat-treated carbon nanofibers (Pyrograf (R) III PR-19, CNF) in a linear low density polyethylene (LLDPE) matrix were assessed. Heat treatment (HT) of carbon nanofibers at 2500 degrees C significantly improved their graphitic crystallinity and intrinsic transport properties, thereby increasing the EM SE of the nanocomposites. Nanocomposites containing 11 vol% (20 wt%) PR-19 HT displayed a DC electrical conductivity of about 1.0 +/- 0.1 X 101 S/m (n = 4), about 10 orders of magnitude better than that of as-received PR-19 CNF nanocomposites. Over a frequency range of 30 MHz to 1.5 GHz, nanocomposites (2.5 mm thick) containing PR-19 HT displayed EM SE average values of about 14 +/- 2 dB (n = 4). Absorption was determined to be the main EM SE mechanism for the heat-treated CNF nanocomposites. The nanocomposites possessed a modulus of 632 +/- 36 MPa (n = 6) (nominally twice that of pure LLDPE) and a strain-to-failure of 180 +/- 98% (n = 6), indicating that a significant ductility is retained in the nanocomposites. Such nanocomposites display potential as absorptive electromagnetic interference shielding materials for thin films and micromolding. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers