AbstractSolid acid composite electrolytes CsH2PO4(CDP)/NaH2PO4(SDP)⋅2H2O were prepared and observed the structural, thermal and transport properties by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray analysis, differential scanning calorimetry, fourier transform infrared spectroscopy, and conductivity measurement. We have investigated superprotonic phase transition at 235°C in CDP, at which the conductivity increased up to 2 to 3 orders of magnitude. The initial dehydration event in CDP occurs at 250°C. The performance of CDP was increased due to the addition of SDP⋅2H2O in the form of stability. Thermal characterization showed that introducing the additivities, dehydration behavior shifted to the lower at the higher temperature. The conductivity is increased above the temperature of 170°C which was found of the composite electrolytes 70CDP/30SDP⋅2H2O, 60CDP/40SDP⋅2H2O. The electrodes were prepared by a vacuum coating unit of silver.