Proton transport in H3PO4- and H2SO4-blended polybenzimidazoles (PBIs) has been studied with both temperature- and pressure-dependent dielectric spectroscopy. The influences of the acid concentration and temperature on the relative conductance and activation volume are discussed. An Arrhenius relation is used to model the temperature-dependent conductivity at a constant acid content. The logarithm of the relative conductance for PBI blended with H3PO4 decreases linearly with increasing pressure. As the temperature increases, the activation volume becomes smaller for PBI blended with H3PO4. It is proposed that proton transport in acid-blended PBI is mainly controlled by proton hopping and diffusion rather than a mechanism mediated by the segmental motions in the polymer. The conductivities of PBIs blended with H3PO4 and H2SO4 are compared. At a 1.45 molar acid doping concentration, the former has the higher conductivity. With water, the conductivity of H3PO4-blended PBI increases significantly.