The structural, electrotransport, and thermodynamic properties of the (1–x)CsH2PO4−xBa(H2PO4)2 system in a wide range of compositions (x = 0.1–0.4) were firstly studied to develop the highly conductive proton electrolytes within the medium-temperature range. At x = 0—0.1, formation of disordered substitutional solid solutions, isostructural to CsH2PO4 (P21/m), with a decrease of the unit cell parameters and considerable increase of proton conductivity as a result of formation of vacancies in the cesium sublattice and weakening of the system of hydrogen bonds, was observed. At x = 0.15–0.4, the heterophase highly conductive systems demonstrating high values of proton conductivity ~10–2 S/cm at x = 0.15—0.2, stable under the long-term isothermal exposures and low humidity (T ~ 200—210°C, RH ~ 15%), are formed. The phase transition disappears, the energy of activation of conductivity decreases from 0.9 to 0.55 eV at x = 0.2. The conductivity of high-temperature phase does not vary with Ba(H2PO4)2 fraction increase to x = 0.2. The mechanisms of transfer of protons were discussed. It has been shown that when x > 0.10 the contribution to proton conductivity of molecules of the water adsorbed on the phase boundary of the composite systems increases.