The electro-oxidative polymerization of benzene in the presence of strong acids yields poly(p-phenylene) efficiently. A strong protonic acid such as trifluoromethanesulfonic acid or Lewis acid such as an aluminum trichloride forms a complex with benzene and reduces the oxidation potential of benzene to 1. 7 V, which brings about the high current efficiency. The acids act both as an electrolyte and as a catalyst in this polymerization of benzene.