In this study, CsH5(PO4)(2)/quartz fiber thin membranes with thicknesses varying from 70 to 150 mu m were prepared by a simple impregnation method and were tested as an electrolyte for fuel cell and electrolytic cell applications. The membranes consisted of a physical dispersion of CsH5(PO4)(2) in the quartz fiber matrix. The crystalline structure and thermal behavior of CsH5(PO4)(2) were not influenced by the quartz fiber. The membrane showed a high conductivity of 33 mS cm(-1) at 180-250 A degrees C under 30% H2O/Ar atmosphere. In addition, the membranes had area-specific resistances of 0.32 and 0.73 a"broken vertical bar cm(2), for corresponding thicknesses of 70 and 150 mu m, which are sufficiently low values compared with those of pellet-type electrolytes. The membrane showed stable conductivity at 220 A degrees C under 30% H2O/Ar atmosphere for 20 h. A fuel cell assembled with the membrane exhibited an open-circuit voltage of 0.93 V and peak power densities of 105 and 72 mW cm(-2), for corresponding thicknesses of 70 and 150 mu m. In addition, ammonia was successfully synthesized from humidified hydrogen and nitrogen under atmospheric pressure in an electrolytic cell assembled with the membrane. An ammonia formation rate of 2.8 x 10(-10) mol cm(-2) s(-1) and a Faradaic efficiency of 0.09% were obtained at 220 A degrees C when the applied voltage was 0.05 V. With the increase of the applied voltage, both ammonia formation rate and Faradaic efficiency decreased rapidly.