The composite polymer electrolytes (1-x)CsH2PO4-xF-2M (x = 0–0.3) have been first synthesized and their electrotransport, structural, and mechanical properties were investigated in detail by impedance, FTIR spectroscopy, electron microscopy, and X-ray diffraction methods. The structure of CsH2PO4(P21/m) with salt dispersion is retained in the polymer electrolytes. The FTIR and PXRD data are consistent, showing no chemical interaction between the components in the polymer systems, but the salt dispersion is due to a weak interface interaction. The close to uniform distribution of the particles and their agglomerates is observed. The obtained polymer composites are suitable for making thin highly conductive films (60–100 μm) with high mechanical strength. The proton conductivity of the polymer membranes up to x = 0.05–0.1 is close to the pure salt. The further polymers addition up to x = 0.25 results in a significant decrease in the superproton conductivity due to the percolation effect. Despite a decrease, the conductivity values at 180–250 °C remain high enough to enable the use of (1-x)CsH2PO4-xF-2M as a proton membrane in the intermediate temperature range.