Poly(tetrafluoroethylene) PTFE/PBI composite membranes doped with H3PO4 were fabricated to improve the performance of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC). The composite membranes were fabricated by immobilising polybenzimidazole (PBI) solution into a hydrophobic porous PTFE membrane. The mechanical strength of the membrane was good exhibiting a maximum load of 35 19 MPa. After doping with the phosphoric acid. the composite membrane had a larger proton conductivity than that of PBI doped with phosphoric acid. The PTFE/PBI membrane conductivity was greater than 0 3 S cm(-1) at a relative humidity 8.4% and temperature of 180 degrees C with a 300% H3PO4 doping level. Use of the membrane in a fuel cell with oxygen, at 1 bar overpressure gave a peak power density of 1 2 W cm(-2) at cell voltages >0 4 V and current densities of 3.0Acm(-2) The PTFE/PBI/H3PO4 composite membrane did not exhibit significant degradation after 50 h of intermittent operation at 150 degrees C. These results indicate that the composite membrane is a promising material for vehicles driven by high temperature PEMFCs. (C) 2009 Elsevier Ltd. All rights reserved.