Proton-conductive composite membranes were prepared from phosphosilicate gel powders and a commercially available polyimide precursor. The conductivity of a composite membrane containing 75 wt.% of gel powder was about 2.5 x 10(-3) S cm(-1) at 180 degrees C under more than 0.4%RH. However, under dry N-2 atmosphere at 180 degrees C, the conductivity decreased with time because of the formation of Si5O(PO4)(6) crystals. The open circuit voltage (OCV) of a fuel cell in which the phosphosilicate gel-polyimide composite sheet was used as electrolyte and Pt-loaded carbon paper sheets were used as electrodes in the membrane electrode assembly. was about 0.85 V at 80 degrees C with a flow of hydrogen and air, and the OCV decreased to about 0.70 V at temperatures higher than 100 degrees C. At 180 degrees C under 2%RH, the cell OCV was 0.67 V; power density of about 14 mW cm(-2) was obtained with current density of 40 mA cm(-2). Consequently, the fuel cell using the composite sheet as an electrolyte was verified to operate at temperatures from room temperature to 180 degrees C. (C) 2008 Elsevier B.V. All rights reserved.