In an effort to develop improved Proton Exchange Membranes for Polymeric Fuel cells two different forms of polyanion inclusion in poly(2,5-benzimidazole) (ABPBI) have been carried out. Namely by (i) sulfonation of pre-formed polymer membranes and (ii) by addition of inorganic phosphomolybdic acid to form hybrid ABPBI–PMo12 membranes. In both cases we have detected an increased proton conductivity of the resulting membranes, associated to an enhanced capacity of the polyanion-modified materials to uptake phosphoric acid (in comparison with previously known ABPBI or even commercial PBI membranes). These new membranes are stable up to 200 °C and feature high conductivities at these temperatures, which makes them promising candidates for higher-temperature PEM Fuel Cells.