A detailed investigation of the polymerisation conditions for poly(2,2′-(1,4-phenylene)5,5′-bibenzimidazole) (para-PBI) in polyphosphoric acid (PPA) led to the preparation of high molecular weight polymer. The polymer solutions could be used directly to produce phosphoric acid (PA)-doped PBI membranes by the PPA process. These membranes showed very high PA doping levels, i.e., up to ∼30 moles of PA per mole of PBI repeat unit. The ionic conductivity of the membranes was 0.24 S cm–1 at 160 °C. Fuel cell performance was measured at various operating conditions by changing temperature, pressure and gases and verified the high carbon monoxide (CO) tolerance of fuel cells operating at this temperature.