Polyethersulfone (PES)-expanded graphite nanocomposites have been prepared by solution blending route after sonicating expanded graphite in dichloromethane. It has been observed that ultrasonication results in nanosheets formation leading to a low percolation threshold of 3wt.%. At 5wt.% filler loading the conductivity is of the order of 10-2S/cm. Hopping type of charge transport occurs at 3.2wt.% expanded graphite in PES below which capacitive effects couple. The effective dielectric constant at low frequency increases with filler concentration. Impedance measurement has been carried out to evaluate interfacial capacitance which, for 3.2wt.% expanded graphite addition in PES, increases to 110pF from 32pF for 1wt.% expanded graphite in the polymer. DSC analysis shows an increment of 12°C in the Tg of PES with 3wt.% expanded graphite suggesting interaction between the polymer and filler. © 2010 Elsevier Ltd.