Fuel cells using a composite sheet composed of phosphosilicate (P2O5-SiO2, P/Si = 1 in mole ratio) gel powder and polyimide operated up to 150degreesC under a relatively low water vapor pressure. Maximum power density of the fuel cell increased with increasing operation temperature up to 150 degreesC and increasing humidity. No degradation was observed even after the continuous operation for 10 h at 150 degreesC and 4% relative humidity (RH) under 50 mA cm(-2). Sieving of the mechanically milled phosphosilicate, gel powders to be less than 5 mum in size was found to improve the homogeneity and thus the proton conductivity of the resultant composite sheets. The conductivity of the composite sheet containing 75 wt.% of the sieved gel powder was 4 x 10(-3) S cm(-3) at 150degreesC and 18% RH. Good contact between the composite sheet as an electrolyte and Pt-loaded carbon paper sheets as electrodes in the membrane electrode assembly (MEA) was very important to attain high performance of the fuel cells, which was achieved by the painting of phosphosilicate sol on the surface of the carbon paper and the hot-pressing of the MEA in vacuo. (C) 2004 Elsevier B.V. All rights reserved.