Anhydrous mixtures of PolyBenzImidazole (PBI) with H3PO4, H2SO4 and HBr have been investigated by Infra Red spectroscopy and impedance measurements. The IR study indicates that the nitrogen of the imide is protonated by the acids. The anions are linked to the polymer by rather strong hydrogen bonding. The conductivity of the acid doped PBI is governed by an activated mechanism (Arrhenius law). Strong correlations between the conductivity and the nature of the associated anions can be established. The best conductivity is achieved with H2SO4. Typically, PBI-3H(2)SO(4) exhibits an anhydrous conductivity of 10(-4) Scm(-1), at 300 K.