A series of composite materials was fabricated by mixing reduced graphene oxide (rGO) powder particles in an epoxy resin. In this paper, we analyze impedance measurements on these materials over broad frequency and temperature ranges. The real and imaginary parts of the effective complex impedance are well fitted to the Cole-Cole equation. The frequency dependence of the ac conductivity follows Jonscher's law with relaxation processes characterized by a broad distribution of relaxation times. The imaginary part of the effective electric impedance collapses onto a single master curve using a single characteristic frequency as a scaling parameter. We find that the electrical properties of the samples are strongly influenced by graphene oxide content. Below percolation threshold, the ac transport can be interpreted as due to electron hopping. Further, we find that the frequency-dependent effective impedance measurements overlap on a single master curve in the range of temperatures explored, showing that a single electrical conduction mechanism is operative. Close and above percolation threshold, the ac conduction originates from both electron tunneling and capacitive paths among the rGO nanoparticles in the polymer bulk.