Electrical conductivity studies of acid doped poplybenzimidazole (PBI) and 85% phosphoric acid have been carried out. The PBI contained about 600 mol% of 85% phosphoric acid and the electrical conductivity was deduced from complex impedance studies which were made at frequencies from 10 to 10(7) Hz. Measurements were made at pressures up to 0.25 GPa and temperatures of about 25, 50 and 75 degrees C. Similar measurements were also carried out on 85% phosphoric acid. At room temperature the electrical conductivity decreases with increasing pressure for both acid doped PBI and 85% phosphoric acid. This behavior is expected for normal liquids and for ions in polymers where the viscosity increases with pressure. The resultant positive activation volume for the acid doped PBI has a magnitude similar to that for polymer electrolytes based on perfluorosulfonic acid containing relatively small amounts of water. Further, as temperature increases, the activation volume becomes smaller. Consequently, the acid doped PBI behaves like a polymer electrolyte where ion transport is mediated by segmental motions of the polymer. Consequently, the acid doped PBI behaves like a polymer electrolyte where ion transport is mediated by segmental motions of the polymer. On the other hand, the activation volume at room temperature for 85% phosphoric acid is much smaller than for acid doped PBI. In addition, at 75 degrees C the electrical conductivity increases with increasing pressure which gives rise to a negative activation volume. Consequently, there is very little similarity between the electrical behavior of the 85% phosphoric acid and of the acid doped PBI which were studied in the present work.