Poly 2,5-benzimidazole (ABPBI) membranes cast from sufficiently high molecular weight polymer were doped with phosphoric acid in order to provide proton conductivity with values ranging between 0.01 and 0.07 Scm-1, depending on the operation conditions. Positive characteristics of these doped membranes are the excellent thermal stability, and acceptable mechanical properties (though weakened after doping). Also, membrane permeability to alcohols is also reduced compared to Nafion®, and no pre-humidification is necessary during fuel cell operation. However, acid doped ABPBI membranes possess certain weak points, such as a higher hydrogen/oxygen gas permeability compared to Nafion®, and a limited resistance to hydroxyl radicals. Nonetheless, satisfactory fuel cell results can be obtained when operating the cells with H2/air, with a maximum power of 170 mW cm-2 at 170°C and atmospheric pressure. © 2011 ECS - The Electrochemical Society.