A proton-conducting H3PO4@MIL-100 inclusion compound was synthesized by incorporating phosphoric acid into a mesoporous chromium(III) trimesate MIL-100 metal-organic framework. The concentration of phosphoric acid in the hybrid system was determined in a wide range of relative humidity using the water vapor sorption measurements of H3PO4@MIL-100. The hybrid compound demonstrates high thermal stability, and the framework retains its original crystal structure as confirmed by Xray powder diffraction and thermogravimetric analysis (TGA) measurements. The proton conductivity at 70 °C and moisture of 5 mol % H2O is 3.0 × 10-2 S·cm-1, which is only 1 order of magnitude lower than that of aqueous phosphoric acid. Importantly, the proton conductivity in a wide range of relative humidity is fully compliant with water content and calculated phosphoric acid concentration in the sample determined by TGA and water vapor sorption measurements. The mechanism of proton conductivity in H3PO4@MIL-100 hybrid compound is discussed.