Phosphoric acid doped ABPBI (poly [2,5-benzimidazole]) membranes were prepared with a new low temperature casting procedure (ABPBI-ET), and their water uptake and proton conductivity were measured. The results were compared with those of ABPBI casted at high temperature (ABPBI-MSA), poly [2-2'-(m-phenylene)-5-5' bibenzimidazole] (PBI), and commercial cross-linked Fumatech ABPBI (ABPBI-C) membranes. The water uptake of ultra thin ABPBI-ET membranes in the range 7-30 nm supported over gold substrate, was also measured and compared with PBI ones. The in-plane proton conductivities of all the membranes were measured in the temperature range from 20 degrees to 120 degrees C at different water activities. The ABPBI-ET doped in 10.6 M H3PO4 and ABPBI-C doped in 14.9 M H3PO4 showed the highest conductivities, even higher than those reported for Nafion membranes. ABPBI-ET membranes showed a maximum as a function of water activity at aw approximate to 0.55, a behavior that can be rationalized in terms of the dissociation constant of the ABPBI-H3PO4 complexes calculated using the Scatchard method, and the H3PO4 concentration in the water inside the membrane. (C) 2016 The Electrochemical Society.