Proton-conducting composite membranes were fabricated by blending Sn0.95Al0.05P2O7 having an excess of phosphates with polybenzimidazole (PBI) and polytetrafluoroethylene (PTFE). The addition of PBI to Sn0.95Al0.05P2O7-PxOy powder stabilized the conductivity of the composite, providing higher conductivities than those of stoichiometric Sn0.95Al0.05P2O7. The addition of PTFE to Sn0.95Al0.05P2O7-PxOy-PBI powder reduced the conductivity but increased the tensile strength. The resulting composite membrane exhibited a conductivity of 0.04 S cm(-1) at 200 degrees C and a tensile strength of 2.30 MPa. Moreover, a fuel cell made with this composite membrane yielded high power densities exceeding 200 mW cm(-2) above 100 degrees C and good durability under unhumidified conditions.