Changes in proton conductivities of phosphosilicate gels derived from tetraethoxysilane and phosphoric acid or triethylphosphate have been discussed on the basis of the structural changes in the gels with the heat treatment. Proton conductivities of both phosphosilicate gels increased with an increase in relative humidity, indicating that continuous paths suitable for fast proton conduction were formed in the gels due to the adsorption of water. Phosphoric acid-derived phosphosilicate gels showed higher proton conductivities than trethylphosphate-derived ones at a given heat treatment temperature in a range from 50 to 600 degreesC. The decrease in the amount of isolated phosphoric acid and the formation of crystalline phases in the phosphosilicate gels lowered the proton conductivity. Phosphoric acid was found to be a preferred starting material to triethylphosphate in order to introduce larger amounts of phosphorus atoms into silica networks without crystallization for the fabrication of highly proton-conductive solid electrolytes. (C) 2001 Elsevier Science B.V. All rights reserved.