The hybrid compounds based on CsH5(PO4)2 and metal-organic coordination framework, Cr-MIL-101 matrix, have been first synthesized. Proton conductivity, structural and thermal properties of (1-x)CsH5(PO4)2–xCr-MIL-101 nanocomposite electrolytes (x = 0.02–0.2) were investigated for the first time. The significant increase of proton conductivity has been shown due to the interface interaction of high dispersed mesoporous Cr-MIL-101 and CsH5(PO4)2. According to X-ray diffraction there is no chemical interaction between the components and no new compounds are formed. The data of X-ray diffraction, differential scanning calorimetry and electron microscopy show significant changes in structural and thermodynamic properties of CsH5(PO4)2 in nanocomposites due to dispersion and amorphization. As a result the proton conductivity of systems that increases by 1–3 orders of magnitude up to 10− 2 S/cm at 130 °C in the low humidity conditions, depends on the composition and goes through maximum at x~0.07–0.11. The conductivity of nanocomposites significantly exceeds the ones for the initial salt and the dispersed matrix. Such new approach affords the solid materials with potent proton conducting properties at moderate temperatures.