Intended for the electrolyte of PEM fuel cell, three poly (2,5-benzimidazole) ABPBI membranes, denoted as Sb-PA, Sn-PA, Fe-PA, are doped with phosphoric acid and one metal phosphate of antimony, tin, and iron. For comparison, another membrane (PA) is doped with an equal amount of phosphoric acid without metal phosphate. Proton conductivities of the Sb-PA, Sn-PA, and Fe-PA membranes are higher than that of the PA membrane at 60-240 degrees C, indicating metal phosphate enhances the membrane conductivity. The enhancement effect is most prominent in Sb-PA, with the maximum conductivity at 160 degrees C, 6.7 x 10(-2) S cm(-1). Mechanically, Sb-PA is also stiffer and stronger than Sn-PA, Fe-PA, PA, with a Young's modulus 390 +/- 60 MPa and a tensile strength 14.3 +/- 0.9 MPa. A H-2/air fuel cell is assembled to prove the Sb-PA membrane is viable for electrolyte application, with the peak power performance 515 mW cm(-2) at 160 degrees C in hydrogen, and 399 mW cm(-2) when its hydrogen fuel is contaminated by 3% carbon monoxide.