ac and electrical properties of carbon-black–epoxy composites measured in the frequency range of 100 Hz to 1 MHz are reported. Experimental results are in remarkable disagreement with the predictions of the statistical percolation theory for real and imaginary parts of the conductivity. They both exhibited, in particular, an abrupt variation at a given fraction of carbon black. The evolution of these transport quantities with the volume fraction of carbon black can only be explained considering a phase transition from a dispersed to an agglomerated state of the conducting fillers. This transition has been successfully verified by optical microscopy of thin polished sections of cured samples. A simple model based on electrostatic repulsion between the carbon-black particles is proposed to interpret such a singular insulator-to-conductor transition.