Previously cast ABPBI membranes were sulfonated by doping with sulfuric acid followed by heat treatment at 450degreesC for 5 min in air. Sulfortation degrees between 35 and 49% of the benzimidazole rings were achieved. The resulting SABPBI membranes were impregnated with phosphoric acid (H3PO4 85%/H2O, 70:30 bath). For concentrated phosphoric acid baths (above 65%), the capacity of these membranes for phosphoric acid uptake (and consequently also their conductivity) increased with the degree of sulfonation. Sulfonated and acid doped SABPBI membranes were characterized in terms of degree of sulfortation, thermal stability (TGA), X-ray diffraction, FTIR spectroscopy and proton conductivity in the dry state, and compared with phosphoric acid impregnated ABPBI studied earlier. The maximum conductivity measured in dry conditions was 3.5 x 10(-2) S cm(-2) at 185 degreesC for SABPBI(.)4.6H(3)PO(4) (with a degree of sulfonation of 41%) which compares favorably with non-sulfonated ABPBI and makes feasible their application in PEM Fuel Cells working at temperatures of 150-200degreesC. (C) 2004 Elsevier Ltd. All rights reserved.