A polybenzimidazole (PBI)/Sn0.95Al0.05P2O7 (SAPO) composite membrane was synthesized by an in situ reaction of SnO2 and Al(OH)(3)-mixed powders with an H3PO4 solution in a PBI membrane. The formation of a single phase of SAPO in the PBI membrane was completed at a temperature of 250 degrees C. Thermogravimetric analysis showed that the PBI membrane was not subject to a serious damage by the presence of SAPO until 500 degrees C. Scanning electron microscopy revealed that SAPO particles with a diameter of approximately 300 nm were homogeneously dispersed and separated from each other in the PBI matrix. Proton magic angle spinning nuclear magnetic resonance spectra confirmed the presence of new protons originating from the SAPO particles in the composite membrane. As a consequence of the interaction of protons in the SAPO with those in the free H3PO4, the H3PO4-doped PBI/SAPO composite membrane exhibited conductivities several times higher than those of an H3PO4-doped PBI membrane at room temperature to 300 degrees C, which could contribute to the improved performance of H-2/O-2 fuel cells. (C) 2011 Elsevier B.V. All rights reserved.