Imidazole and 1-methyl imidazole (Me-Im) were used as additives in polybenzimidazole (PBI) equilibrated with phosphoric acid (PA), a system shown to be a high-temperature proton-conducting polymer electrolyte. The influence of different concentrations of this additive on the conductivity of these membranes was measured by a four-probe conductivity measurement, at temperatures in the range of 80-200 degreesC, under various humidity conditions. Correlation was found between the conductivity of liquid solutions of concentrated phosphoric acid and that of H3PO4 in the PBI membranes.