Preparation and characterization of membranes of poly(2,5- benzimidazole) (ABPBI) (as thin as 20 mum) for polymer electrolyte membrane fuel cells are reported. These membranes were prepared by solution casting and then impregnated in phosphoric acid baths. Their characterization included thermogravimetric analyses, conductivity measurements, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. These membranes have high thermal stability and good proton conductivity at temperatures up to 200 degreesC (6.2 x 10(-2) S cm(-1) at 150 degreesC and 30% relative humidity), similar to the well-known commercial polybenzimidazole (PBI). ABPBI membrane-electrode assemblies (MEAs) using commercially available E-TEK Pt/C electrodes were tested in H-2/O-2 cells, obtaining power densities similar to those of PBI MEAs studied here as reference at temperatures of up to 180 degreesC, humidifying the reactants at room temperature. (C) 2004 The Electrochemical Society.