K2HPO4-KH5(PO4)(2)-Ce0.9Gd0.1P2O7 (K-CGP) composites are synthesized and their ionic conductivity is measured in 110-230 degrees C for application as electrolytes in proton-conducting ceramic-electrolyte fuel cells (PCFCs). The K-CGP composites are prepared by mixing of 10 wt% powder of different K2HPO4-KH5(PO4)(2) mixtures with Ce0.9Gd0.1P2O7 (CGP) and heat-treating in 250-350 degrees C range. The phase analysis of various composites by X-Ray diffraction (XRD) shows that K2HPO4 and KH5(PO4)(2) are present as amorphous phase. The scanning electron microscopy (SEM) is performed to analyze the microstructure of various K-CGP composites. Electrochemical impedance spectroscopy (EIS) is performed at different temperatures to obtain the ionic conductivity in humidified atmosphere (water vapor pressure, pH(2)O = 0.12 atm) in 110-230 degrees C range. The ionic conductivity of the composites is >0.01 S.cm(-1) in 110-190 degrees C range which is significantly higher than that of CGP. Among various compositions of K-CGP composites, one with K2HPO4/KH5(PO4)(2) molar ratio of 5:5 and heat-treated at 300 degrees C shows maximum ionic conductivity of 0.04 S.cm(-1) at 150 degrees C. It is observed that the K-CGP composites are stable around the temperature corresponding to their maximum ionic conductivity but at the elevated temperatures >= 190 degrees C they become porous, because of the dehydration as well as the leaching out of the molten KH5(PO4) 2 phase. (C) 2015 The Electrochemical Society. All rights reserved.