Poly [2.2'-(p-phenylene)-5,5'-bibenzimidazole] (pPBI) is synthesized by mixing 3,3'-diaminobenzidine tetrahydrochloride and terephthalic acid in polyphosphoric acid (PPA) at 240 degrees C for 12 h. pPBI can be cast in membrane form directly from a polymerization solution (direct casting) or from a methanesulfonic acid solution (MSA casting). Membranes obtained by direct casting are almost amorphous and have much higher acid doping levels and larger water uptake values than those prepared by MSA casting. As a result, the pPBI membrane from direct casting has a high proton conductivity of 0. 15 S cm(-1) (at 180 degrees C and 1 % relative humidity) at an acid doping level of 8.78 mol H3PO4 per mole of polymer repeat unit. Moreover, this membrane has good mechanical stability (tensile strength of 57 MPa at 22 degrees C and 24% relative humidity) even at this high doping level. (C) 2007 Elsevier B.V. All rights reserved.