An effective resistance of solid acid/phosphate composites was reduced by fabricating their thin-film electrolyte membranes for fuel cells operating at 100-300 degrees C. Solid acid and phosphate serve as an ionic conductor and supporting matrix, respectively, in these composites. Three-types of porous matrices were synthesized on a Pd film substrate by the electrostatic spray deposition technique, and then the solid acid was soaked under reduced pressure. The thin-film composite electrolytes showed almost the same conductivity in a wide temperature range of 100-200 degrees C, regardless of the difference in matrix microstructure. Above 200 degrees C, however, the microstructure of matrix significantly affected the thermal stability of the thin-film composite. The composite consisting of the matrix with the reticular structure, characterized by a three-dimensional interconnected porous network, achieved high thermal stability as well as low area specific resistance. Fuel cells employing thin-film membrane electrode assemblies were successfully operated at 200 degrees C, and the electrochemical measurements clarified the improvements. (C) 2011 Elsevier B.V. All rights reserved.